Gunsmithing Choking the muzzle bore on a rimfire barrel?

[MtnGhost]

I've been machining barrel blanks for a few years now with pretty solid results. Mainly rimfire and lately airgun slug barrels, but I have been wanting to experiment more with adding choked muzzles on some of the barrels that I suspect would shoot much tighter groups if the last inch and change of the bores had a slight taper.

Has anyone here done anything like that before? I thought that I had an old gunsmithing book that talked about this, but there's not a lot of information out there (I'm sure the barrel manufacturers aren't motivated to share how they do it).

That being said, I have been able get pretty close to the desired result with some of the airgun barrels when I thread the muzzles in the standard 1/2-20 UNF or M14x1 (depending on the platform). I can manipulate the bore a little, usually when turning down the barrel material taking heavier cuts towards the muzzle end as I turn it down about the desired angle that I want the bore to taper off to. I'm just not sure if there's a better way to do this, especially with some of the rimfire barrel projects, where I don't have much material to work with.

Hopefully this makes sense. Any tips would be greatly appreciated!

 

[AirGunShawn]

I recently saw a choke for a air rifle barrel for sale. He said he had used it two different ways, one simply by tightening the bolts and
the other in a hydraulic press. It was basically two blocks of steel similar to a clamp collar except it was rectangular in shape. So imagine two blocks of steel that were one inch thick by 1.5 tall by 4" long. He wanted 225.00 for it and said he had done many air-gun barrels with it. Davis is a well known air-gun guy so it may have worked. I can also think of other mechanical Greenlee punches that use the nut and bolt approach to punch holes in steel panels. They have a thrust bearing under the bolt heads to keep the screw heads from galling the punch block. Personally I would add some thrust bearings under the heads if I were to go that route. They are cheap from McMaster Carr. Given the cost of matl. and bearings you wouldn't have much lost if it were not successful. Probably less than $20.00 As I recall the Greenlee punches used high grade socket head screws with the bearings and hardened washers. That said, your blocks would have through holes in them. You could always double up the nuts if needed. You could probably find them on the net. They are electricians tools. All you need is say .001"

Look up Greenlee punches for a better idea. In fact you may buy a set and just make the blocks. !?

I've been machining barrel blanks for a few years now with pretty solid results. Mainly rimfire and lately air-gun slug barrels, but I have been wanting to experiment more with adding choked muzzles on some of the barrels that I suspect would shoot much tighter groups if the last inch and change of the bores had a slight taper.

Has anyone here done anything like that before? I thought that I had an old gunsmithing book that talked about this, but there's not a lot of information out there (I'm sure the barrel manufacturers aren't motivated to share how they do it).

That being said, I have been able get pretty close to the desired result with some of the airgun barrels when I thread the muzzles in the standard 1/2-20 UNF or M14x1 (depending on the platform). I can manipulate the bore a little, usually when turning down the barrel material taking heavier cuts towards the muzzle end as I turn it down about the desired angle that I want the bore to taper off to. I'm just not sure if there's a better way to do this, especially with some of the rimfire barrel projects, where I don't have much material to work with.

Hopefully this makes sense. Any tips would be greatly appreciated!

 

[MtnGhost]

I recently saw a choke for a air rifle barrel for sale. He said he had used it two different ways, one simply by tightening the bolts and
the other in a hydraulic press. It was basically two blocks of steel similar to a clamp collar except it was rectangular in shape. So imagine two blocks of steel that were one inch thick by 1.5 tall by 4" long. He wanted 225.00 for it and said he had done many air-gun barrels with it. Davis is a well known air-gun guy so it may have worked. I can also think of other mechanical Greenlee punches that use the nut and bolt approach to punch holes in steel panels. They have a thrust bearing under the bolt heads to keep the screw heads from galling the punch block. Personally I would add some thrust bearings under the heads if I were to go that route. They are cheap from McMaster Carr. Given the cost of matl. and bearings you wouldn't have much lost if it were not successful. Probably less than $20.00 As I recall the Greenlee punches used high grade socket head screws with the bearings and hardened washers. That said, your blocks would have through holes in them. You could always double up the nuts if needed. You could probably find them on the net. They are electricians tools. All you need is say .001"

Look up Greenlee punches for a better idea. In fact you may buy a set and just make the blocks. !?

That's a great idea. At one point I had envisioned some kind of beefy tapered collet in a hydraulic chuck, but this approach opens up some options. I have two specific barrel ODs that are the most common for the airgun barrels that I work with (14mm and 15mm), but the rimfire barrels are anywhere from 0.592" to the 0.92" Shillen.

I'll have to try this out soon with some scrap barrels and get a feel for it. I could mill out some blocks that size, but I have a friend with some big CNC machines who I would probably pay if I needed anything exotic lol

I appreciate the feedback!!!

 

[skl1]

My understanding is that most of the guys who do this lap the choke into the barrel. So they're actually opening up the (usually slightly undersized) portion of the barrel back from the choked area.

 

[AirGunShawn]

Here is a Greenlee punch and die set. The thrust bearing is under the head of the bolt on the lower half. If you look them up, you will see how they use a small hydraulic hand pump to pull the die together but it only saves time. Since everything deflects and you only want to squeeze .002"
or less you will want to be certain that your blocks are very ridged. If you look into maximum deflection at load, you will find that steels that
have a higher tensile strength still deflect about the same. At least from memory.... It's something like WLcubed / 48EI I would crush it in a press
if you have one on hand. Please let me know what happens.

That's a great idea. At one point I had envisioned some kind of beefy tapered collet in a hydraulic chuck, but this approach opens up some options. I have two specific barrel ODs that are the most common for the airgun barrels that I work with (14mm and 15mm), but the rimfire barrels are anywhere from 0.592" to the 0.92" Shillen.

I'll have to try this out soon with some scrap barrels and get a feel for it. I could mill out some blocks that size, but I have a friend with some big CNC machines who I would probably pay if I needed anything exotic lol

I appreciate the feedback!!!

 

[alamo5000]

While I don't know specifically about a choke there is a company that was making tapered bores. I had a great conversation with them about this topic several years ago now.

Their bore say started at 22 cal and ended up I think at 20 cal or something like that.

According to my recollection of the conversation they said groups were not impacted one way or the other. All of that was still dependant on the usual factors. Where it made a big difference though was in extended ranges.

 

[AirGunShawn]

Try the first article. The second may be good reference or it may be simpler to purchase. There are various rollers both flat and crowned
if you wanted to try making a rolling compressor. Maybe three rollers, two on one side and one on the other. Say an altered vice jaw that holds two rollers and the opposite jaw holding one roller. You could close the vice while you spin the barrel. Conceptually.
The article claims the barrel matl is dead soft and will collapse fairly easily. Are you using SS matl ??

I'm not positive if my CZ 457 MTR barrel is chocked but it measures .860 except for the last 2 1/2" that transitions to .859"

Why choke a barrel? | Pyramyd Air Gun Blog

B.B. blog post titled Why choke a barrel? explores new information, history, tips and tricks at the Pyramyd Air.

www.pyramydair.com

Airgun barrels

www.lothar-walther.com

 

[nikonNUT]

You might look at something like a Parker Gates hose crimper. Not sure of the clamping force but they go to 10000 psi and have multiple collets. There are electric and hand pump versions. With 8 points of contact it might offer a nice even squeeze?

 

[AirGunShawn]

The hose crimp sounds like an interesting idea. How would you imagine that you limit it's crush on the barrel?
Is the multi piece collect deep enough to put a hardened steel plug in the crimper to limit it's crush? I'm guessing that he will
need an inch of choke. Also are the replaceable inserts stable in the unit or is there a chance that they close on a taper if no plug is used.
Maybe you simple stall the unit out if it is hydraulic.

You might look at something like a Parker Gates hose crimper. Not sure of the clamping force but they go to 10000 psi and have multiple collets. There are electric and hand pump versions. With 8 points of contact it might offer a nice even squeeze?
View attachment 7702926

 

[AirGunShawn]

The first two pieces go together. It's a Parker multi piece hose crimper. It simply drives the segmented cone into a tapered female to
push the insert together. If you had this exact set-up you could likely control the crush by limiting the amount of travel the press was allowed to collapse. Maybe some 1x2x3 blocks and some shim stock.

The third phot is a rough concept of a multi roller system. Almost all bearing companies have hardened roller assemblies that you could try out.
Some of the rollers have shafts and some use through holes. They all seem doable but only you know what equipment you have on hand.

If you go with the Greenlee clamp block approach, two thrust bearing at $20.00 each from McMaster carr will be needed. Say a 5/8"hole and part number 60415K12. It is shown in the fourth photo. For the three roller system shown in photo number three you may look into::

High-Load Shaft-Mount Track Rollers at McMaster Carr​

One-Piece Thrust Ball Bearings​

Shaft-Mount Track Rollers​

I want to see your first proto type on Tuesday..... LoL

Shawn

 

[nikonNUT]

The hose crimp sounds like an interesting idea. How would you imagine that you limit it's crush on the barrel?
Is the multi piece collect deep enough to put a hardened steel plug in the crimper to limit it's crush? I'm guessing that he will
need an inch of choke. Also are the replaceable inserts stable in the unit or is there a chance that they close on a taper if no plug is used.
Maybe you simple stall the unit out if it is hydraulic.

I "think" crush would be controlled by a) compressed diameter of the collet and b) the amount of hydraulic pressure applied. A micrometer or I.D gage of some sort would be required as each barrel might behave differently. As for a stop, I think I would go with something dead soft that could be clamped or mounted on the barrel 1" below the crown. Barrel goes in from below and hits against the collet. As pressure is applied the collet and stop move in unison maintaining the distance to be compressed.

 

[MtnGhost]

WOW, great ideas!!!! Reading the responses chronologically, I guess I should have noted that I work with a couple of specific barrels that are a great match with some specific high BC projectiles, but the bore diameters are a thou or two too large to be absolutely perfect for the ammo. Can't really lap the aggressively just short of where the choke should be in these cases, but when I threaded them the groups tightened right up, but it's too hard to get the chokes tapered just right this way (and not everyone wants a muzzle thread lol).

That little hydraulic press looks AWESOME!!! That drawing of the 3 roller bearings around the barrel also gives me an idea! I had a little benchtop Grizzly 7x16 once upon a time, and I was turning some polymer rods that the brass in my steady rest kept tearing into. I made some inserts drilled out 1/4" and attached some roller bearings. I couldn't clamp them down too tight since it would leave an imprint of the roller, but I would bet that it could work with some finesse.

That said, the press approach is probably the best of all. Controlling the taper could be a challenge, using a tapered bore plug to form the taper could be the way to go, maybe using PEEK or Delrin? Just thinking along the lines of being able to tap it out afterwards. There's gotta be a good sound solution! lol

 

[nikonNUT]

So in the case of that press the ram is solid (it might be drilled and tapped to mount the ring that presses the collet). You could measure the depth subtract the amount of crimp you want and create a stop that is the difference plus a nipple to fit in the bore for locating. Downside... With that style of the stop in place you might not be able to "pop" the barrel to break the wring between the collet and the collar that compresses it. My though.... Make a puck out of Delrin, POM, PEEK or UHMW urethane (think cutting board). Make it the same dimension as mentioned before plus say a 1/4". Then spot face the center 1/4" deep at the diameter of the barrels you use to get the dimension back. Locate it in the collar of the press with an o-ring or the such. Barrel goes in, locates in the divot, run press to crimp, and retract. Puck moves away, giving room to push the barrel breaking the taper lock. Pros are nothing goes in the bore that could get stuck. This is all based on the press that @AirGunShawn showed as it looks like it runs the collet up against a flat plate (I think) so the muzzle of the barrel is pointing down into the press instead of coming up from underneath. Just thinking out loud....Pay me no mind!

 

[Mr.BR]

While the straw thin Air rifle barrels are mechanically chocked, rimfire barrels are not they typically have a larger diameter muzzle end than the rest of the barrels which induces a slight choke.

One of the reasons you should never stick a tapered contour barrel on a rimfire. Nor thread it to small diameter threads like 1/2-20

Muzzle Threading — Don’t Remove Too Much Steel « Daily Bulletin

Our friend Robert Whitley of ARX Enterprises LLC has learned, through careful measurement and testing, that some barrels threaded 5/8 x 24 tpi at the muzzle may not deliver optimal accuracy. The reason is that the end of the barrel can bell out slightly, like a trombone, because too much steel...

bulletin.accurateshooter.com

Contouring influences the bore diameter, so the best and simplest way to choke a barrel is to make it fat at the muzzle. That is why RF BR scene moved to smaller tennons ( not to induce a choke just after the chamber end)


Depiction of bore change due to external machining , ,figure B shows the enlarged bore under the threaded part , figure C outside taper causes reverse tapering of the bore.

Reverse taper barrels are one way to taper the bore and tune the muzzle end in Sporter classes that are not allowed tuners

 

[AirGunShawn]

Mr. BR, Would you please school me on inducing a choke. Using my Anschutz 54, it sounds like your saying that they made the last 1.5"
a larger diameter to induce a choke.
Since my Vudoo is threaded 1/2-28, you make it sound like it would have greater accuracy if it were not.
Please keep it simple so I can follow the line of thought.

 

[Mr.BR]

Yes its a smart way of harnessing the metals ''behaviour'' when being machined, In any case it is a whole lot easier to control the final bore size via machining the final barrel dimensions after the bore is cut then as it is being cut which is the other way to do it .

this is not some anecdote but a cold hard fact. More material is removed outside , more the bore opens up, check the above link from the accurate shooter .

1/2-28 thread is rather a small diameter,it means that the bore opened up where metal is turned away for the thread,so accuracy could be adversely impacted, much more in case of Rimfire than centerfire.
If you have the barrel threaded go for the largest possible thread diameter you can .

 

[Greg Langelius *]

In Powder driven barrels, longer term use causes a mild taper to develop within the bore. The breech end becomes slightly wider in diameter. IMHO, it is reflection of the pressure differentials between the muzzle and breech ends; the the greater the pressure, the greater the obturation, and the potential for bore wear.

It was an argument in favor of setting back a barrel and cutting a new chamber. The rechambered barrels, with their taper, usually shot better/tighter than in their original condition. Getting this to work requires that the original rifling is only minimally worn and undamaged.

Greg

 

[Mr.BR]

In Powder driven barrels, longer term use causes a mild taper to develop within the bore. The breech end becomes slightly wider in diameter. IMHO, it is reflection of the pressure differentials between the muzzle and breech ends; the the greater the pressure, the greater the obturation, and the potential for bore wear.

It was an argument in favor of setting back a barrel and cutting a new chamber. The rechambered barrels, with their taper, usually shot better/tighter than in their original condition. Getting this to work requires that the original rifling is only minimally worn and undamaged.

Greg

wut?

BR folks literally change barrels that would typically last 2-3000Rds , often after 800rds as barrel accuracy drops past its peak and there is no way that you can rechamber anything shoot to any degree, to shoot better.

Setting back a barrel is only about getting some more mileage out of worn 'pipe' , fine if you want to DIY it but often no worth the effort if you are paying a gunsmith as in the end you are paying near new barrel cost, for a barrel that is way past its prime .

 

[Seymour Fish]

Mr. BR, Would you please school me on inducing a choke. Using my Anschutz 54, it sounds like your saying that they made the last 1.5"
a larger diameter to induce a choke.
Since my Vudoo is threaded 1/2-28, you make it sound like it would have greater accuracy if it were not.
Please keep it simple so I can follow the line of thought.

Air gun, Some say threading a cut-rifled muzzle does not change bore diameter, wheras it may Expand the bore diameter of a buttoned bbl. perhaps offset by screwing on a tightly fitting device. Old school hand taper-lapped vs new school cut rifled bores with zero taper : the bench rest guys lean heavily to the former at short range, and never thread the muzzle.

 

[AirGunShawn]

Mr. BR
It sounds like your saying that if I drill, ream and ball burnish a perfectly round hole in a piece of 2" diameter 4140 round stock. it will stay .50" diameter. But then if I go back and use a single point tool to reduce it's diameter to 1.25" then the bore is going to get bigger by some dimension. Or is it the other way around. My Ans 54 has a bigger muzzle than the rest of the barrel. Is that extra .12" diameter somehow constricting the muzzle creating a choke? In any case, I'm just hoping for an explanation that makes sense to my tiny brain. Greg's opinion makes sense on it's face since everything is malleable. I should offer up that I have machined for a living and spent 30 years doing machine design so any formula that you offer up should be no problem.

Shawn

 

[AirGunShawn]

I've surfed the net and thus far found nothing about IDs growing when the ODs are reduced. I have heard people say this kind of thing before and wondered if they were thinking of CRS. CRS is different than HRS in that CRS has a lot of internal stress built in during the manufacturing process.

I recently asked someone to support their product and how they used a certain technology and I was referred to this abstract.https://core.ac.uk/reader/9068840 on Particle Impact Damping. I ask what he had incorporated into his unit to gain the benefits of PDT
and I got the answer of " look at the publication from Texas A&M " I was able to see in his design and the abstract that he had the absolute minimum to attempt to utilize PID /PDT. It may or may not work but saying even NASA engineers Looked at his device doesn't give me confidence.
I gave him a really common device that most people could use and get a feel for it even if they didn't understand the smoke and mirrors.

Not looking to be adversarial but just need pure facts and not what the best BR shooter in the US thinks.

 

[Mr.BR]

I've surfed the net and thus far found nothing about IDs growing when the ODs are reduced. I have heard people say this kind of thing before and wondered if they were thinking of CRS. CRS is different than HRS in that CRS has a lot of internal stress built in during the manufacturing process.

Not looking to be adversarial but just need pure facts and not what the best BR shooter in the US thinks.

It happens no matter if the barrel is cut or button rifled. Doesn't even need to be a barrel, literally any bore will change with machining the OD is considerable enough ,depending on the amount of stress left in the material.

I don't know why you would have an issue finding anything when i literally posted you a link where its demonstrated in layman's terms
This is simple way to demonstrate as you can gauge your bore pre and post-turning, its not a question if it happens,its more a question how fine graduated your gauge pins are to be able to measure this as changes in ID are extremely small.

Muzzle Threading — Don’t Remove Too Much Steel « Daily Bulletin

Our friend Robert Whitley of ARX Enterprises LLC has learned, through careful measurement and testing, that some barrels threaded 5/8 x 24 tpi at the muzzle may not deliver optimal accuracy. The reason is that the end of the barrel can bell out slightly, like a trombone, because too much steel...

bulletin.accurateshooter.com

 

[AirGunShawn]

So his last 6mm barrel with 3/4 threads was a constant 6mm fit down it's length.
All well and good but with no explanation of why it happens.
And it still feels like it would be fine if the bore were cut after the contour.

 

[Mr.BR]

No its not constant 6mm bore down the lenght , change in ID is just too small to measure with a gauge pin graduation on hand, because the OD cut was less ,but change in ID still took place.

This is literally taken from Bartlein barrels FAQ page ,they lay it out close enough but ,take the BS about no stress in cut rifled barrel lightly ,yes proces induces less stress into the steel, but is still there from basic steel rolling even when stress relieved , if it was that simple to get rid of stress in metals no one would bother with fancy tool steels or Meehanite castings made stress relived to be as stress-free as they can for use in machine building.

Long story short this is known phenomenon and European rimfire barrel makers used this to make chokes in the barrels for past 70+years. Anschutz is just perhaps best known for it.

Does fluting do anything for accuracy?​

Depends on how the barrel is made. I would never flute a button rifled barrel. Why? Button rifling induces a lot of stress into the steel. Any secondary machining work like fluting, contouring of the barrel especially where you make a drastic contour change or even just cutting and crowning the muzzle. If you hit a residual stress point in the steel the machining operation will relieve the stress and this can cause the bore to go sour (open up on you). The last place you want it to go sour is right at the muzzle. Even though a button rifled barrel typically gets restressed relieved again after button rifling there are no guarantees. Also no barrel maker can measure for residual stress in the material. We do single point cut rifling and don’t induce and stress in the steel. So machining flutes into the barrel we don’t run into the problem with the bore opening up/going sour like you do with a button rifled barrel. We do have minimums as to the depth of the flutes. I do feel you can flute a barrel to deep and cause harmonic and vibration problems which can effect accuracy.

FAQs | Bartlein Barrels

www.bartleinbarrels.com

 

[Seymour Fish]

So his last 6mm barrel with 3/4 threads was a constant 6mm fit down it's length.
All well and good but with no explanation of why it happens.
And it still feels like it would be fine if the bore were cut after the contour.

The simple answer to your confusion is to do the test yourself. 2 cut rifle barrels/2 buttoned. Say 1.25”. Contour he’ll out of one of each. Measure the ID’s and report back

 

[Shootin Stuff]

Have you thought of adding a heavily shrink fit collar to the end of your barrel? You might need to experiment with barrel profile (wall thickness) and amount of interference fit once normalised to get it to work, but I have no doubt it would work with some experimentation. You could even muck around with tapered bores in your collar to achieve various rates of taper.

Edit: similar to the anshutz posted above.

 

[Rabid Skunk]

Hey, i'm new to this forum

I started and owned Airhog Pneumatics until I sold the business and retired some six (6) years ago.
Am well aquanted with choked air rifle barrels, am in the process of making a Carbon fiber rimfire barrel from a 22 air rifle slug barrel.
The barrel i'm using is a FX smooth twist X .22 barrel liner. The barrel ends I am using is stainless and plan to set it up like a Dan Wesson revolver, that is where it can put the liner under tension so it can be adjusted for resonance of what ever rimfire ammo I get.
If anyone has any input let me know, I don't know everything.

 

[MtnGhost]

Hey, i'm new to this forum

I started and owned Airhog Pneumatics until I sold the business and retired some six (6) years ago.
Am well aquanted with choked air rifle barrels, am in the process of making a Carbon fiber rimfire barrel from a 22 air rifle slug barrel.
The barrel i'm using is a FX smooth twist X .22 barrel liner. The barrel ends I am using is stainless and plan to set it up like a Dan Wesson revolver, that is where it can put the liner under tension so it can be adjusted for resonance of what ever rimfire ammo I get.
If anyone has any input let me know, I don't know everything.

Hmm well I build PCP airguns and have had the .22cal FX STX liners shooting 40gr RBT slugs at rimfire velocities (around 2900PSI was the max that I have tested). I've also engineered some pretty insane barrel housing and tensioning systems (alleviating the need for liner compression).

That stainless is pretty soft and fairly thin. I definitely wouldn't run hot loads in it. Second issue is the bore/groove diameter. The .218 is usually the largest that those tight bores will shoot. Are you reloading your own rimfire ammo?

 

[Rabid Skunk]

The reason for stainless is to prevent anodic corrosion problems with the carbon fiber/stainless contact surfaces. 24K. working pressure wont be a problem as long as I stay within SAMMI specs. And the bushing is an interference fit and sized to fit the action ring. As far as bore diameter, by the time it’s lapped with 5 micron diamond paste it should spec out fine except the choke which which will be the .002” less.
My main business was subcontracting with Sandra Labs, Ames Research, Lawerence Livermore on experimental projects, even Disney on a couple of times, all to do with design and building high pressure gas delivery systems. My secondary business was being the USA distributor for Falcon airguns out of Birmingham UK.

 

[Rabid Skunk]

The reason for stainless is to prevent anodic corrosion problems with the carbon fiber/stainless contact surfaces. 24K. working pressure wont be a problem as long as I stay within SAMMI specs. And the bushing is an interference fit and sized to fit the action ring. As far as bore diameter, by the time it’s lapped with 5 micron diamond paste it should spec out fine except the choke which which will be the .002” less.
My main business was subcontracting with Sandra Labs, Ames Research, Lawerence Livermore on experimental projects, even Disney on a couple of times, all to do with design and building high pressure gas delivery systems. My secondary business was being the USA distributor for Falcon airguns out of Birmingham UK.

 

[elwarpo]

It would be interesting to use the Gerlich gun principle. Squeeze bore to get higher velocity

 

[Rabid Skunk]

A .22 air rifle barrel is choked down at the muzzle end by about .002 starting at about 2 inches from the crown. The FX smooth twist X barrel has the rifling pressed in and does not look like normal rifling, (see Picture) the bore diameter is constant up to the last two (2") then tapers down about .002" This bore desig fit thus reducing barrel drag, allowing the projectile to gain greater velocity than a normal barrel. the final 2 inches of choke give stability and accuracy to the slug.
From experience from building, repairing and shooting them i've found them to be quiet accurate, sub .5 groups at 50 yards not uncommon. From my understanding from my friends still in the industry that, the newer airgun slug barrels are better than the choked pellet shooting barrel------------------------------

 

[Rabid Skunk]

 

[MtnGhost]

A .22 air rifle barrel is choked down at the muzzle end by about .002 starting at about 2 inches from the crown. The FX smooth twist X barrel has the rifling pressed in and does not look like normal rifling, (see Picture) the bore diameter is constant up to the last two (2") then tapers down about .002" This bore desig fit thus reducing barrel drag, allowing the projectile to gain greater velocity than a normal barrel. the final 2 inches of choke give stability and accuracy to the slug.
From experience from building, repairing and shooting them i've found them to be quiet accurate, sub .5 groups at 50 yards not uncommon. From my understanding from my friends still in the industry that, the newer airgun slug barrels are better than the choked pellet shooting barrel------------------------------

Having spent an embarrassing amount of time over the last few years tinkering with FX barrels and doing airgun-related R&D, I definitely commend you on the creativity for repurposing an FX liner for rimfire. I have all tools and experience to lap bores within reason of my target bore diameters, but I've just worn out more FX liners than I care to share running them in the Taipan Veteran PCPs with the 40gr Griffin RBT slugs. The CZ's and some of the polygonal LW's with custom twist rates are just a far better platform to build around IME.

With the FX liners, as soon as I start seeing streaks of pitting along the apex of the bore diameter / lands (anywhere from an inch to 2-3" before the choke and all the way through the choke), I know that their days are numbered. I keep my ammo very clean and properly lubed, liners sleeved with proper unidirectional CF tubing, and some other "safe" harmonic tuning techniques that I can't get into right now, but this phenomenon has happened to me (and with many other liners belonging to others shooting slugs at high power), and it has happened with both the factory compression nut system and my own adjustable tensioned-based system.

I've had somewhere north of 100 FX liners on my bench, but out of them all I had a PRIZED 600mm factory choked slug A liner that came out of an older FX Crown belonging to popular YT airgunner. Sold the rifle and kept that liner, running it in several different rifles with no issues whatsoever. 1/2" at 100y when I did my part all day long with a variety of ammo.

When I made some big performance breakthroughs with a prototype PCP valve system, I eventually started using that 600m liner with 40gr .217 RBT slugs in the R&D rifle. 3 months later, the liner was absolute trash. I also dedicated another liner for shooting a different 40gr .217 FB with a 2S nose, and like clockwork - it only lasted 3-4 months.

Now the crazy thing that puzzles me is that I can run 34gr ammo with a 2S nose (made from the SAME soft Corbin lead) in the FX liners and the liners hold up well. Working pressure is almost always the same regardless of the projectile, but I don't know why the larger slugs eat the liners up so quickly.

I have also removed the chokes one some damn good shooting liners in testing (using a couple of 700mm slug and pellet liners and a 600mm slug A), but the only real benefit was not having to size down some of the .218-2228" slugs that I had. I never really got any of them shooting better than they were with .217" ammo.

Last couple of notes that I can share with you regarding FX liners - if you can get a hold of some of the older pellet liners with the 19" or 23-24" twists, they would probably work out very well for rimfire ammo with high transonic - low supersonic velocities, so long as you remove that little section of free bore and ream them out right for your ammo. I probably don't have to tell you this, but freebore jump with anything but lighter, diabolo style pellets is NOT a good thing to contend with in FX liners.

Sorry to long wind all of that. I didn't really want to dive deep at first, but those have been my experiences.

 

[Rabid Skunk]

Having spent an embarrassing amount of time over the last few years tinkering with FX barrels and doing airgun-related R&D, I definitely commend you on the creativity for repurposing an FX liner for rimfire. I have all tools and experience to lap bores within reason of my target bore diameters, but I've just worn out more FX liners than I care to share running them in the Taipan Veteran PCPs with the 40gr Griffin RBT slugs. The CZ's and some of the polygonal LW's with custom twist rates are just a far better platform to build around IME.

With the FX liners, as soon as I start seeing streaks of pitting along the apex of the bore diameter / lands (anywhere from an inch to 2-3" before the choke and all the way through the choke), I know that their days are numbered. I keep my ammo very clean and properly lubed, liners sleeved with proper unidirectional CF tubing, and some other "safe" harmonic tuning techniques that I can't get into right now, but this phenomenon has happened to me (and with many other liners belonging to others shooting slugs at high power), and it has happened with both the factory compression nut system and my own adjustable tensioned-based system.

I've had somewhere north of 100 FX liners on my bench, but out of them all I had a PRIZED 600mm factory choked slug A liner that came out of an older FX Crown belonging to popular YT airgunner. Sold the rifle and kept that liner, running it in several different rifles with no issues whatsoever. 1/2" at 100y when I did my part all day long with a variety of ammo.

When I made some big performance breakthroughs with a prototype PCP valve system, I eventually started using that 600m liner with 40gr .217 RBT slugs in the R&D rifle. 3 months later, the liner was absolute trash. I also dedicated another liner for shooting a different 40gr .217 FB with a 2S nose, and like clockwork - it only lasted 3-4 months.

Now the crazy thing that puzzles me is that I can run 34gr ammo with a 2S nose (made from the SAME soft Corbin lead) in the FX liners and the liners hold up well. Working pressure is almost always the same regardless of the projectile, but I don't know why the larger slugs eat the liners up so quickly.

I have also removed the chokes one some damn good shooting liners in testing (using a couple of 700mm slug and pellet liners and a 600mm slug A), but the only real benefit was not having to size down some of the .218-2228" slugs that I had. I never really got any of them shooting better than they were with .217" ammo.

Last couple of notes that I can share with you regarding FX liners - if you can get a hold of some of the older pellet liners with the 19" or 23-24" twists, they would probably work out very well for rimfire ammo with high transonic - low supersonic velocities, so long as you remove that little section of free bore and ream them out right for your ammo. I probably don't have to tell you this, but freebore jump with anything but lighter, diabolo style pellets is NOT a good thing to contend with in FX liners.

Sorry to long wind all of that. I didn't really want to dive deep at first, but those have been my experiences.

Hi, am aware of the problems with the liners as you mentioned. From observation the free bore is to tight for slugs and the transition is to abrupt. My proposed solution is the use 5 micron diamond paste to inlarge the freebor and

Having spent an embarrassing amount of time over the last few years tinkering with FX barrels and doing airgun-related R&D, I definitely commend you on the creativity for repurposing an FX liner for rimfire. I have all tools and experience to lap bores within reason of my target bore diameters, but I've just worn out more FX liners than I care to share running them in the Taipan Veteran PCPs with the 40gr Griffin RBT slugs. The CZ's and some of the polygonal LW's with custom twist rates are just a far better platform to build around IME.

With the FX liners, as soon as I start seeing streaks of pitting along the apex of the bore diameter / lands (anywhere from an inch to 2-3" before the choke and all the way through the choke), I know that their days are numbered. I keep my ammo very clean and properly lubed, liners sleeved with proper unidirectional CF tubing, and some other "safe" harmonic tuning techniques that I can't get into right now, but this phenomenon has happened to me (and with many other liners belonging to others shooting slugs at high power), and it has happened with both the factory compression nut system and my own adjustable tensioned-based system.

I've had somewhere north of 100 FX liners on my bench, but out of them all I had a PRIZED 600mm factory choked slug A liner that came out of an older FX Crown belonging to popular YT airgunner. Sold the rifle and kept that liner, running it in several different rifles with no issues whatsoever. 1/2" at 100y when I did my part all day long with a variety of ammo.

When I made some big performance breakthroughs with a prototype PCP valve system, I eventually started using that 600m liner with 40gr .217 RBT slugs in the R&D rifle. 3 months later, the liner was absolute trash. I also dedicated another liner for shooting a different 40gr .217 FB with a 2S nose, and like clockwork - it only lasted 3-4 months.

Now the crazy thing that puzzles me is that I can run 34gr ammo with a 2S nose (made from the SAME soft Corbin lead) in the FX liners and the liners hold up well. Working pressure is almost always the same regardless of the projectile, but I don't know why the larger slugs eat the liners up so quickly.

I have also removed the chokes one some damn good shooting liners in testing (using a couple of 700mm slug and pellet liners and a 600mm slug A), but the only real benefit was not having to size down some of the .218-2228" slugs that I had. I never really got any of them shooting better than they were with .217" ammo.

Last couple of notes that I can share with you regarding FX liners - if you can get a hold of some of the older pellet liners with the 19" or 23-24" twists, they would probably work out very well for rimfire ammo with high transonic - low supersonic velocities, so long as you remove that little section of free bore and ream them out right for your ammo. I probably don't have to tell you this, but freebore jump with anything but lighter, diabolo style pellets is NOT a good thing to contend with in FX liners.

Sorry to long wind all of that. I didn't really want to dive deep at first, but those have been my experiences.

My

 

[Rabid Skunk]

What I have seen in several friends FX barrels, for what it's worth, it appears to me that the abrupt transition at the lead end of the choke is slowly leading up. I have seen this problem with a few choked LW barrels and they had abrupt choking. I've order a bore scope to look at one I have laying around here, If I am right, I plan on lapping that area to increase the length of the choke transition, hopefully it will work.

 

[MtnGhost]

What I have seen in several friends FX barrels, for what it's worth, it appears to me that the abrupt transition at the lead end of the choke is slowly leading up. I have seen this problem with a few choked LW barrels and they had abrupt choking. I've order a bore scope to look at one I have laying around here, If I am right, I plan on lapping that area to increase the length of the choke transition, hopefully it will work.

Yeah they definitely lead up and sometimes the lands in the choke aren't very square with one another. I made the investment in a Hawkeye Pro-slim bore scope with the extra 35 & 90 degree attachments, and I was pretty horrified when I first examined some of my FX liners lol.

Anyways I wouldn't go too overboard with lapping the choke since the lands are more like a polygonal barrel than traditional flat lands. I've had really good results with pseudo-firelapping pellet barrel chokes to shoot slugs (primarily .22 and .30 LW non-poly barrels), but it's only made FX liners shoot worse.

 

[Rabid Skunk]

Yeah they definitely lead up and sometimes the lands in the choke aren't very square with one another. I made the investment in a Hawkeye Pro-slim bore scope with the extra 35 & 90 degree attachments, and I was pretty horrified when I first examined some of my FX liners lol.

Anyways I wouldn't go too overboard with lapping the choke since the lands are more like a polygonal barrel than traditional flat lands. I've had really good results with pseudo-firelapping pellet barrel chokes to shoot slugs (primarily .22 and .30 LW non-poly barrels), but it's only made FX liners shoot worse.

What method did you use in your attempt to lap the transition, did you make a cast slug of the rifling for the lap?

 

[MtnGhost]

What method did you use in your attempt to lap the transition, did you make a cast slug of the rifling for the lap?

Yeah it was a real pain in the ass. I made a traditional brass jag threaded to a good cleaning rod that had nylon bushings around it to keep it concentric. Poured lead in from the muzzle and pulled it out from the breech to add my polishing compounds (I used the 3.5 micron compound from the jeweler's polishing compound set that I bought from a Canadian company and finished with a 0.25). The muzzle end of the jag had a brass compression screw that I made with a little flanged base so the lead can be compressed back to the bore/groove diameters from the choke to unchoked area.

 

[Rabid Skunk]

Yeah it was a real pain in the ass. I made a traditional brass jag threaded to a good cleaning rod that had nylon bushings around it to keep it concentric. Poured lead in from the muzzle and pulled it out from the breech to add my polishing compounds (I used the 3.5 micron compound from the jeweler's polishing compound set that I bought from a Canadian company and finished with a 0.25). The muzzle end of the jag had a brass compression screw that I made with a little flanged base so the lead can be compressed back to the bore/groove diameters from the choke to unchoked area.

I have used Cerrosafe which is harder than lead however, upon casting it shrinks by .002 then within 1 hour it expands to the actual bore size.
The way I handled it was to wait for 30~40 minutes then start lapping, if it starts to feel to tight, stop and remelt and cast a new lap as you move along. The idea behind this is two fold, the first is that it is much harder and deforms less thus giving a better shape of the rifling. Second is that while using its gradual expansive quality I can work from the from the breach end starting at the muzzle and work backwards towards to the breach. First starting with 6 micron then clean the bore and change to 3 micron then on to 1 micron, cleaning the bore in between the differing diamond grits. Another nice thing is you can cast the whole barrel and remove it in one piece then, wait an hour and it will be an exact copy of your bore that you can mic anywhere along it, to see where you are at.
Used this technique with my Ruger SRH 9" 454, after working up a load it groups about 1 inch @ 30 yards out of a Ransom Rest.

 

[MtnGhost]

I have used Cerrosafe which is harder than lead however, upon casting it shrinks by .002 then within 1 hour it expands to the actual bore size.
The way I handled it was to wait for 30~40 minutes then start lapping, if it starts to feel to tight, stop and remelt and cast a new lap as you move along. The idea behind this is two fold, the first is that it is much harder and deforms less thus giving a better shape of the rifling. Second is that while using its gradual expansive quality I can work from the from the breach end starting at the muzzle and work backwards towards to the breach. First starting with 6 micron then clean the bore and change to 3 micron then on to 1 micron, cleaning the bore in between the differing diamond grits. Another nice thing is you can cast the whole barrel and remove it in one piece then, wait an hour and it will be an exact copy of your bore that you can mic anywhere along it, to see where you are at.
Used this technique with my Ruger SRH 9" 454, after working up a load it groups about 1 inch @ 30 yards out of a Ransom Rest.

Hmmm I've never tried to cast a full barrel with Cerrosafe. Do you think it could be done with a 600-700mm .22cal FX liner?

 

[E. Bryant]

Me most of the time: Yeah, I know some stuff about guns

Me reading this thread:

 

[PlinkIt]

Proof of concept seems like it should be straight forward in my simpleton brain

Turn the barrel end

Turn an interference fit ring
(Maybe of something with a lower thermal expansion factor like inconel? Doubt you need to though...)

Liquid nitro the barrel end / heat the ring

Assemble and see if it makes a difference...

Seems like you could make many different tests...
Apply difference in interference on various rings
Apply taper...
Different lengths...

If any of the patterns really show an improvement then perhaps investing in some type of tapered mandrel for a hammer forged barrel set up would be an idea

 

[Rabid Skunk]

Hmmm I've never tried to cast a full barrel with Cerrosafe. Do you think it could be done with a 600-700mm .22cal FX liner?

Hmmm I've never tried to cast a full barrel with Cerrosafe. Do you think it could be done with a 600-700mm .22cal FX liner?

I cannot say about .17 barrels but do know it works with a 22 & 25 cal. Just remember that you push it out from the muzzle end. You have to rotate it along the rifling axis during removal.
If you are unable to loosen it, most likly the rifling is irregular and you may want to think about replacing that barrel. If all else fails, you can always heat the barrel with a hair drier and melt it out; Cerrosafe melts under 190F.