Action receiver/barrel thread length

[JB.IC]

Is there a reason why some actions use a thread length of say 1.5” long for the receiver/barrel thread length and another action using only 0.68”?

Context is this would be for a 33XC or 300Norma sized cartridge.

Is there any positives or negatives to the longer or shorter thread length. Attached is a picture of what I’m referring to if I’m not being specific enough.

 

[Supersubes]

Probably not any practical benefits to going longer. The tenon lengths are just a reflection of the tradeoffs that were made in design. Consider the actions that use the shorter/standard tenons are all basically Remington footprint clones. There’s only so much room there for tenon thread if the action face remains in the Remington location. Extending that would guarantee that the action won’t drop in easily to a Remington stock. Ga precision’s house action for a time left the recoil lug in the Remington location, but extended the action face forward to make a longer tenon. They may still do that, I don’t know. I can’t see why AI went so long with theirs but it did later allow for adding the Quick-lock barrel retention feature to the newer guns. That wouldn’t have been possible with a short tenon. Speaking of AI, Ga precision makes their Tempest action. It’s basically an AI bolt in a mostly Remington footprint. Very similar bolt head and a breech ring to the AI, which needs quite a bit of space. They left the recoil lug in the Remington location and extended the action face forward, but only enough for about .800” (from memory) worth of tenon.

There are a few large benchrest exactions that have non-standard style tenons. I’m sure they’re looking for maximum rigidity with very large/long straight cylinder barrels.

For large diameter cases, tenon diameter and not length is more important imo. 1.062” is standard, and quite a few manufacturers offer 1.1” tenons. It’s a small amount larger, but a step in the right direction. Going larger leaves too little meat for the receiver threads. Adding diameter to the receiver ensures it won’t drop in to a Remington stock anymore.

Sorry for the long post, but this stuff is interesting to me.

 

[JARHEAD1371]

I've wondered the same thing. I went with a Surgeon 591 that's tendon length is just under an inch. If I were to design an action, it would have a thread length of at least an inch. If nothing else, it would make me feel better when shooting a heavy barrel and suppressor.

On a side note, I've always ordered barrels that match the diameter of the barrel shank to match my action. The typical shank is 1.2-1.25 and my Surgeon is 1.35. In my mind, the extra shoulder helps with rigidity but it might not actually do anything. If it helps or not, I think the final look is much better.

Has anyone done any actual testing of this?

 

[JB.IC]

I’d like to know more. I get the OD being bigger is required for larger cartridges.
the length is the thing that I didn’t know for sure. I guess all you really would need is enough thread length to ensure proper torque to the receiver. Maybe 0.68” is enough for 75ft-lbs

 

[Supersubes]

I’d like to know more. I get the OD being bigger is required for larger cartridges.
the length is the thing that I didn’t know for sure. I guess all you really would need is enough thread length to ensure proper torque to the receiver. Maybe 0.68” is enough for 75ft-lbs

75ft lbs is absolutely nothing for a thread that large, even considering the delicate nature of barrel and receiver thread vs a carbon steel nut/ bolt combo. In fact maximum torque for a nut and bolt that large, made of Nylon, is about 70 ft lbs. The lug nuts on your truck are likely 75-90 ft lbs, and they’re only 7/16” or so in diameter. Max torque for a 1.062-16 thread (nut and bolt) is a little north of 1100 ft/lbs. Since we’re talking about barrels and receivers though, torque numbers will be way down from that figure. Also important to note that the first few threads closest to the shoulder carry most of the load. Adding threads would just add unnecessary length to the package. I don’t think you’d be fixing anything by going longer.

Here’s a stress analysis pic pulled from the web with its caption from the writer.

 

[JB.IC]

75ft lbs is absolutely nothing for a thread that large, even considering the delicate nature of barrel and receiver thread vs a carbon steel nut/ bolt combo. In fact maximum torque for a nut and bolt that large, made of Nylon, is about 70 ft lbs. The lug nuts on your truck are likely 75-90 ft lbs, and they’re only 7/16” or so in diameter. Max torque for a 1.062-16 thread (nut and bolt) is a little north of 1100 ft/lbs. Since we’re talking about barrels and receivers though, torque numbers will be way down from that figure. Also important to note that the first few threads closest to the shoulder carry most of the load. Adding threads would just add unnecessary length to the package. I don’t think you’d be fixing anything by going longer.

Here’s a stress analysis pic pulled from the web with its caption from the writer.
View attachment 7356703

thanks for the info! That makes a lot of sense and the graphic will help any marines that are lost in the sauce

 

[Steel head]

75ft lbs is absolutely nothing for a thread that large, even considering the delicate nature of barrel and receiver thread vs a carbon steel nut/ bolt combo. In fact maximum torque for a nut and bolt that large, made of Nylon, is about 70 ft lbs. The lug nuts on your truck are likely 75-90 ft lbs, and they’re only 7/16” or so in diameter. Max torque for a 1.062-16 thread (nut and bolt) is a little north of 1100 ft/lbs. Since we’re talking about barrels and receivers though, torque numbers will be way down from that figure. Also important to note that the first few threads closest to the shoulder carry most of the load. Adding threads would just add unnecessary length to the package. I don’t think you’d be fixing anything by going longer.

Here’s a stress analysis pic pulled from the web with its caption from the writer.
View attachment 7356703

Some of the torque recommendations on similar sized bolts in heavy equipment applications would make 75# seem hand tight.

I’ve been searching and studying about tenons a bit as I wanna build up a long barrel 37XC.

 

[Safety_3rd]

Reviving an old thread. I was looking at prefits for my new terminus zeus and noticed how little actual threads there are compared to that of the ruger precision or the impact (bottom left). Given that I plan to hand tighten my zues then use the side bolts to torque down on sides to retain it, I was curious if the explosion of the round would be sufficient to cause the barrel to move ever so slightly while rounds were fired. In reading the above "the first seven threads" do most of the work, and it looks like there are seven total so I presume this is good to go, however my hesitance is that its only hand tight and so any slop in the threads isnt removed by the torque of tightening it down.
Please excuse all the tabs below... I'm trying to find powder and bullets

 

[Stoweit]

I’ve not heard of one problem with Terminus barrels loosening up. Seems like a non issue to me.

 

[9x23w]

Kirby Allen once wrote to me that he would never build his Raptor super magnum on a Ruger No. 1 because the tenon length was too short. So maybe there's some minimum length that matters?

 

[Edds]

When chambering a large diameter cartridge like a 300 Norma on a barrel that has a 1.062" diameter tenon, short is good.

 

[9x23w]

"...short is good."

Just curious why you think so. It would seem that a longer tenon would support the barrel better. The Steyr SSG's, for example, have longer tenons than the sporting versions of the same era.

 

[Supersubes]

"...short is good."

Just curious why you think so. It would seem that a longer tenon would support the barrel better. The Steyr SSG's, for example, have longer tenons than the sporting versions of the same era.

There are competing issues here, but large diameter cases cause a lot of movement of the chamber at pressure(radial expansion). The area not threaded has more meat around the chamber. This is why some receivers use a larger diameter tenon. On the other hand, those larger cases have more bolt thrust as well, which puts greater shear load on the threads.

 

[surgeon260]

Not an expert by any means, but Robert Gradous once explained to me that only the first few tenon threads of the barrel actually engage the receiver in a load bearing way, so no strength or rigidity is gained by a longer tenon. Intuitively it makes sense to me that reducing the diameter of the barrel steel by threading it can actually make it less rigid.
Those who worry about tenon length may be overthinking it.

 

[samuraiwarrior23]

Guys,is this why inside my action has a several exposed threads ?

 

[JB.IC]

@badassgunworks did you post anything about this topic?

 

[badassgunworks]

Gladly, First of all whatever size tennon you run . it needs to be a perfect fit to action. This is why i hand lap barrel to action. Second, what ever cartridge you use you need to a tennon wall that is at least .250 or thicker per side. The larger the shank the better the higher the tpi. the better 18 or 20 is better then 16 tpi due to thread undercut. Third point, some people think the longer tennon is better for longer barrels. I question that here is why if your threads are properly fitted and your shoulder on barrel is square to action face and its torqued to lets say 60 to 90 lbs the matched faces wont allow any movement. There is more of a issue then a gain with this theory. For example if your using for example the xc cartridge being aprox. 3.050 long with a longer tennon you have more of the over all length of the cartridge in a weaker section of your chamber and if your barrel is not hand lapped for a tight fit to your action as support does not help. Your better off having a higher % of your over all cartridge in the shank portion of your chamber not tennon. This is why i never condone savage nut barrels as you loose the support of the larger shank. When it comes to cartridge and chamber selection in X action and barrel over kill is your best option.

 

[badassgunworks]

There are competing issues here, but large diameter cases cause a lot of movement of the chamber at pressure(radial expansion). The area not threaded has more meat around the chamber. This is why some receivers use a larger diameter tenon. On the other hand, those larger cases have more bolt thrust as well, which puts greater shear load on the threads.

Bolt thrust is dictated by case taper. case diameter in relation to taper. and overall body length in relation to taper. There are many things besides case surface that dictates the amount of Bolt thrust for example column diameter in relation to undercut of extractor Groove. Some bolt thrust is 100% optimal when sharing and displacing pressures to help share the overall induced stress load. Case head design Corner radius and stress risers by Design also cause bolt thrust. there are other factors that I will not go into right know as well. At end of study of hydro Dynamics will answer any and all questions in regard to case design

 

[Rubicon Precision]

Here’s a stress analysis pic pulled from the web with its caption from the writer.
View attachment 7356703

I know this is an old thread, but I came across it google searching for something else.
Regarding that stress analysis above, I would assume that is based on the fastener being tensioned to about 80% of the yield strength. I am curious what the analysis would look like with a typical barrel tension, maybe 5%-10% of the yield strength. Would the engagement be more evenly distributed, and a longer tenon actually be more stable?

 

[Supersubes]

I know this is an old thread, but I came across it google searching for something else.
Regarding that stress analysis above, I would assume that is based on the fastener being tensioned to about 80% of the yield strength. I am curious what the analysis would look like with a typical barrel tension, maybe 5%-10% of the yield strength. Would the engagement be more evenly distributed, and a longer tenon actually be more stable?

Thats a good question, and I'm not certain, but I suspect it is still the case that the first few threads take the load. I did some tests early on using dykem which showed the same pattern. Not definitive though since it would be difficult for me to prove out exactly what helix angle my lathe cuts for a given thread vs what the receiver is cut to.

Anyone know a mechanical engineering student looking for a project?

 

[moosemeat]

Gladly, First of all whatever size tennon you run . it needs to be a perfect fit to action. This is why i hand lap barrel to action. Second, what ever cartridge you use you need to a tennon wall that is at least .250 or thicker per side. The larger the shank the better the higher the tpi. the better 18 or 20 is better then 16 tpi due to thread undercut. Third point, some people think the longer tennon is better for longer barrels. I question that here is why if your threads are properly fitted and your shoulder on barrel is square to action face and its torqued to lets say 60 to 90 lbs the matched faces wont allow any movement. There is more of a issue then a gain with this theory. For example if your using for example the xc cartridge being aprox. 3.050 long with a longer tennon you have more of the over all length of the cartridge in a weaker section of your chamber and if your barrel is not hand lapped for a tight fit to your action as support does not help. Your better off having a higher % of your over all cartridge in the shank portion of your chamber not tennon. This is why i never condone savage nut barrels as you loose the support of the larger shank. When it comes to cartridge and chamber selection in X action and barrel over kill is your best option.

Are you taling about hand lapping threads? I would think that would do nothing because if the threads fit together at all to be lapped, lapping is just removing material and making them looser. Also, the final testing place of the threads, the torqued position is where it stops and would not be changed much from lapping but all threads ahead of that would be looser, possibly making the final threads that recieved the least "lapping" hold proportionately more load.
I am actually asking with the intent to learn and not to bash. I am not a fun smith but work in a industry where cutting threads to tolerances that are tighter than class 3 is common.
I also dont know if a tighter thread would be beneficial because the barrel is going to index on the shoulder which if cut correctly should provide the desired cylindricity.
Please explain to me where I'm wrong because I am sure I missed something because I am not a gunsmith or mechanical engineer. I just make shit out of metal.

 

[badassgunworks]

Are you taling about hand lapping threads? I would think that would do nothing because if the threads fit together at all to be lapped, lapping is just removing material and making them looser. Also, the final testing place of the threads, the torqued position is where it stops and would not be changed much from lapping but all threads ahead of that would be looser, possibly making the final threads that recieved the least "lapping" hold proportionately more load.
I am actually asking with the intent to learn and not to bash. I am not a fun smith but work in a industry where cutting threads to tolerances that are tighter than class 3 is common.
I also dont know if a tighter thread would be beneficial because the barrel is going to index on the shoulder which if cut correctly should provide the desired cylindricity.
Please explain to me where I'm wrong because I am sure I missed something because I am not a gunsmith or mechanical engineer. I just make shit out of metal.

It is common in the firearm industry to hand lap threads with 1200 grit to have a perfect fit yes you can cut threads close to perfect but lapping makes them perfect it done with a little tighter tollerances NOT cut loose (almost too tight to screw togather by hand ) it creates a perfict fit i also lap barrel recoil lug to action and barrel shoulder even after are both trued

 

[Baron23]

Ga precision’s house action for a time left the recoil lug in the Remington location, but extended the action face forward to make a longer tenon.

As does the Defiance Deviant Elite, right? Looks like they kept everything the same but extended the action forward of the recoil lug, no?

Deviant Elite - Defiance Machine | Custom Bolt Rifle Actions

Like the Deviant Tactical, the Deviant Elite has an integral scope base and recoil lug and is perfect for competition.

defiancemachine.com

 

[Supersubes]

As does the Defiance Deviant Elite, right? Looks like they kept everything the same but extended the action forward of the recoil lug, no?

Deviant Elite - Defiance Machine | Custom Bolt Rifle Actions

Like the Deviant Tactical, the Deviant Elite has an integral scope base and recoil lug and is perfect for competition.

defiancemachine.com

Ya, same thing. There might be minor technical differences like a bolt on rail for the templar iirc. Defiance made Ga’s Templar series of action since all the way back before they were called Defiance machine(Phoenix machine I think).

 

[Mech Eng]

Just recently I set up an excel sheet for calculating thread pull out for a project at work and what it really comes down to is the shear engagement of the thread. One up side to a long thread length is that if the engagement loose it could help with a barrel RTZ. But I was wondering the same thing when I was designing my own action.

 

[Ledzep]

Typical threaded fastener design is premised around clamping, with not a ton of thought typically given to alignment. Clamping force and stiffness of the clamped members is what the engineering books teach about. Usually for a bolt an nut of the same/similar material and condition you balance out the tensile strength of the bolt vs. the thread shear force of the engagement around 5-7 threads... If one or the other (bolt/nut) is significantly weaker, you can run into instances where significantly more threads are needed to balance out the thread strip vs. tensile limit teeter totter....

But here with rifles we have a hollow bolt (chamber), we're nowhere near tensile limit of the barrel, nor the shear stress for the threads, and while we might be curious about clamping force and stiffness, I think anecdotally anyway, it doesn't seem to matter for our shooting performance metrics...

About the only thing I can think of would be "wheel base" for alignment to the receiver, but this could be accomplished with 5 threads for clamping, a long space of cylinder (unthreaded), and then a conical/tapered interface with the receiver's front end... I tend to think this is categorized in the "theoretically better" file bin... In the real world, my ARC Nucleus 1.0 with its sandwiched lug and .685" tennon held onto a 37" MTU .300 PRC barrel with no problems to report other than it was too fucking long and awkward for convenience.

This reminds me of the fitment of the barrel and receiver on Swiss K31 rifles. There are two cylinders, a smaller one towards the breech, a larger one towards the receiver face (I think the crazy fuckers ground these to very precise diameters). In between them is a threaded portion with a clearance/undercut gap on either side of it, then all the way at the front of the interface is a shoulder. I don't know if it matters, but you can see pretty clear intent that each piece had its roll to play and was separate from the other features. Two cylinders for alignment, threads and shoulder for clamping.

Picture shamelessly stolen from the internet:

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[BadAccountant]

TacOps rifles are torqued WAY the hell on, with some rumors quoting 500 lb. ft. of torque. That's on a factory 700 action worked over by his shop, so I wouldn't worry about the strength of the thread.

 

[Mech Eng]

Like @Ledzep said it is a balancing act between stripping threads or breaking a bolt. So I did a calculation for a 1.0625 x 20 this class 3 bolt and found the balance point of 0.8768" thread length. And I really like that k31 shank. And I know @Supersubes already put a picture but here is one from fastenal.

 

[alpine_troop]

Super interesting mechanical info guys! I had pondered this question myself

 

[Rubicon Precision]

Regarding that K31 shank pictures above, that follows pretty much my thoughts. BUT, I think it would need to be a light interference fit or greater for it to make a real “RTZ”. The fit would need to be interference, yet be allowed to be assembled/disassembled with the threaded portion.
My non-academic educated mind feels the apposing 60 degree boy/girl thread angles would achieve the same thing by forcing a center/return to center, even if we pretend the threads beyond 7X the pitch were floating in air.

Could be wrong. But I do know if you thread a barrel in 7 turns and measure the sag/play at 26”, it’s going to be a whole lot more than when the barrel is 14 turns into the receiver.

 

[Mech Eng]

Super interesting mechanical info guys! I had pondered this question myself

If you haven't already got a copy of Machinery's Handbook, I would suggest getting a copy as it has tons of information in it. You can find a PDF for free if you look.

Regarding that K31 shank pictures above, that follows pretty much my thoughts. BUT, I think it would need to be a light interference fit or greater for it to make a real “RTZ”. The fit would need to be interference, yet be allowed to be assembled/disassembled with the threaded portion.
My non-academic educated mind feels the apposing 60 degree boy/girl thread angles would achieve the same thing by forcing a center/return to center, even if we pretend the threads beyond 7X the pitch were floating in air.

Could be wrong. But I do know if you thread a barrel in 7 turns and measure the sag/play at 26”, it’s going to be a whole lot more than when the barrel is 14 turns into the receiver.

When I first started working as a machinist I was taught that threads were for joining/clamping and pins were for location position. But those guys were old school tool and die makers. I really like that K31 shank setup but wonder why no one does a taper shoulder.

 

[Ledzep]

*cough*

 

[hero's machine]

Not to dig up the past but I ran across an action that I've had in my shop for about 20 years. When I got my first lathe capable of cutting metric threads I failed to swap back the gears from metric to imperial. I made a clean up cut on the tenon and went back make a finish cut....it obviously made a mess.

Which leads me to the question that there's obviously some who are qualified to answer.

What is the minimum thickness for the action before yielding to torque and how thick before it no longer makes any difference.

Brnchrest actions being sleeved makes sense and mountain hunting actions being slimmed down makes sense....but is there a standard where enough is enough?

Thanks.