Gunsmithing Polishing a new chamber?

[Rprecision]

The last 3 rifle's I chambered, both with SS and CM barrels, were left raw as the finish reamer left them. I have yet to encounter a problem with this method.

I have been reading some on the topic of polishing chambers. There seems to be 2 schools of thought about the topic. To be clear this is not discussing trying to eliminate dimensional problems such as pitting, scoring, etc. This is how a new chamber is finished, primarily for a bolt action rifle.

1. Leave it alone. This maintains the tightest dimensions. Break the sharp corners at the entrance, but no internal polishing is done to the chamber wall.

2. Polish it to varying levels ranging from very light to mirror polish. Those of a benchrest persuasion tend to favor a high polish. Others a light polish to break the smooth surface left from reaming.

The main discussion surrounding this topic is the relationship between the brass case and the chamber, specifically how well they grip each other during firing. It is proposed a medium or course polish leaves a rough surface for the brass to grip. This in turn reduces bolt thrust which is safer and improved action longevity. Alternatively, its argued a mirror polish has more grip, is more attractive, improves brass life and so on. There is clearly a broad range here

I have never polished one. I am considering these philosophies but dont have any experience polishing. Im inclined to try a medium scotch bright IF I decide to try anything.

My application is hunting guns, match and target guns.

What do you do and why? What can you share, suggest, etc.

Thanks

 

[AIAW]

I can attest to having witnessed severe bolt thrust with extremely polished chambers. I am a firm believer that a bit of crosshatch in the chamber is essential for expansion gripping.

A mirror finish can (not always though) create a suction like affect also that makes primary extraction very difficult.

I am sure a professional gunsmith here can truly confirm this though.

 

[BurnOut]

I know that LRI has a specific (non-polished) finish that they do on their chambers for the specific reason of allowing the case to "grab" the chamber under pressure. I'm sure that Chad will be along at some point to offer his input regarding the "why's" behind this approach.

 

[Rprecision]

I know that LRI has a specific (non-polished) finish that they do on their chambers for the specific reason of allowing the case to "grab" the chamber under pressure. I'm sure that Chad will be along at some point to offer his input regarding the "why's" behind this approach.

Thanks for the information, I hope he has something to add to the topic. I always find his information insightful.

 

[Ledzep]

I know that LRI has a specific (non-polished) finish that they do on their chambers for the specific reason of allowing the case to "grab" the chamber under pressure. I'm sure that Chad will be along at some point to offer his input regarding the "why's" behind this approach.

Yeah it's called put some 320 grit on a stick and move it back and forth. VERY SPECIFIC!

I've done as-cut, 320, 400, 600, and 800 grit. Never really noticed any difference in my rifles, but I have seen similar problems as noted above with mirror polished or as-cut chambers that seem to have primary extraction issues. Anymore I hit it with 400 grit usually for about 20-30 seconds just to put something on the surface. Lots of ways to skin the cat.

 

[Hellbender]

Here's more than you ever wanted to know about chamber polishing....

Rifle Chamber Finish & Friction Effects on Bolt Load and Case Head Thinning Calculations done with LS-DYNA

 

[Rprecision]

Here's more than you ever wanted to know about chamber polishing....

Rifle Chamber Finish & Friction Effects on Bolt Load and Case Head Thinning Calculations done with LS-DYNA

That was a interesting read! I'm not sure what to make of it, its counterintuitive

 

[Hellbender]

I wrap a piece of 320 wet/dry around a chamber fitting bore mop and spin it with a drill.

 

[cwinner]

Here's more than you ever wanted to know about chamber polishing....

Rifle Chamber Finish & Friction Effects on Bolt Load and Case Head Thinning Calculations done with LS-DYNA

I agree it's was an interesting read........high polish for brass life and rough finish for lower bolt thrust is what I took away....... thinking I'll stay in the middle and continue a quick touch up with 400 grit wrapped in a dowell immediately after chambering, seems to be working fine for me.

 

[AIAW]

I don‘t get how they are equating brass life with chamber polish though. If you have good headspace then this isn’t a problem in the slightest.

Even for virgin unfired brass this is solved with common fire-forming techniques.

That testing seems a little ehhh to me. Scotch-Brite green for me and call it good.

 

[supercorndogs]

I think what it is showing is that the brass slides to make the case head meet the bolt. Rather than the brass sticking to the chamber, and the case head stretching to meet the bolt. I think the longer brass life is just theoretical, and something you would never see in practice if you are bumping shoulder correctly.

 

[Ledzep]

And in the world of fatigue life, I'll take trashed brass before a cracked bolt/receiver 100% of the time.

 

[Hellbender]

I've seen (many times) while shooting matches in a hard rain (everything from .223 to 6.5 Creed, even a .308), guys popping primers and showing extreme pressure when their ammo and/or gun's chamber is wet. I've seen this even with factory ammo.

Can't imagine how just lower case wall friction, by being wet, could do that, but it does.

I went to the 320 grit to give more "grab" many years ago, and I've never had the problem, but I'm probably also more careful than the average guy to keep stuff dry after seeing this happen many times ( I also don't load to the ragged edge like many guys).

As you know, a popped primer will usually blow a Rem. 700 extractor, ruin a firing pin and/or a Jewell trigger (which were more common a few years ago when I was shooting heavy).

I also saw an AI go down with a blown case in a hard rain shooting factory 6.5 Creed ammo at a match in MO.

 

[sandwarrior]

I've seen (many times) while shooting matches in a hard rain (everything from .223 to 6.5 Creed, even a .308), guys popping primers and showing extreme pressure when their ammo and/or gun's chamber is wet. I've seen this even with factory ammo.

Can't imagine how just lower case wall friction, by being wet, could do that, but it does.

I went to the 320 grit to give more "grab" many years ago, and I've never had the problem, but I'm probably also more careful than the average guy to keep stuff dry after seeing this happen many times ( I also don't load to the ragged edge like many guys).

As you know, a popped primer will usually blow a Rem. 700 extractor, ruin a firing pin and/or a Jewell trigger (which were more common a few years ago when I was shooting heavy).

I also saw an AI go down with a blown case in a hard rain shooting factory 6.5 Creed ammo at a match in MO.

I'm wondering if what you are seeing is the dynamics of the water not moving during the sudden (as nearly as possible) and high (1 ATM to 3742 ATM) rise of pressure creating a blockage rather than acting as a lubricant? I know that will happen when water is in the barrel.

I also gotta say bolt thrust is also affected by cartridge taper. Any of the Mauser cases and especially the .22-250/.250 Sav. have a lot of taper and that lost grip in the chamber translates to heavy boit thrust. On the other hand the Creedmoor cases have minimal taper and grip the chamber walls better

One thing I didn't see in the test (or didn't understand) was how much outward force during firing was naturally occuring between the case wall and the chamber wall.

 

[Steel head]

I don‘t get how they are equating brass life with chamber polish though. If you have good headspace then this isn’t a problem in the slightest.

My thoughts as well.
proper headspace and chamber finish is a win/win.

 

[supercorndogs]

I've seen (many times) while shooting matches in a hard rain (everything from .223 to 6.5 Creed, even a .308), guys popping primers and showing extreme pressure when their ammo and/or gun's chamber is wet. I've seen this even with factory ammo.

Can't imagine how just lower case wall friction, by being wet, could do that, but it does.

I went to the 320 grit to give more "grab" many years ago, and I've never had the problem, but I'm probably also more careful than the average guy to keep stuff dry after seeing this happen many times ( I also don't load to the ragged edge like many guys).

As you know, a popped primer will usually blow a Rem. 700 extractor, ruin a firing pin and/or a Jewell trigger (which were more common a few years ago when I was shooting heavy).

I also saw an AI go down with a blown case in a hard rain shooting factory 6.5 Creed ammo at a match in MO.

Liquid is not compressible, therefore it effectively reduces chamber size.

 

[gnochi]

Especially with titanium receivers, go for the rougher chamber. Lug setback is the main failure mode.

 

[mram10]

I prefer to polish. 400 wet with cutting oil on a pencil, 600 wet and finish with wet red scotchbrite. No adverse effects in my big calibers (338 edg, 300rum, etc). BR argue this all the time and never come to a conclusion. Bolt thrust theory is a whole nuther topic

 

[Kadams1563]

You mean there is more to this then just cramming green scotch brite down the chamber while spinning on the lathe? That's how it gets done around here and never a problem.

 

[cwinner]

I think what it is showing is that the brass slides to make the case head meet the bolt. Rather than the brass sticking to the chamber, and the case head stretching to meet the bolt. I think the longer brass life is just theoretical, and something you would never see in practice if you are bumping shoulder correctly.

Agreed.... proper headspace and loading technique will likely effect brass life much more......... it might be a fun project to rebarrel a couple take off blanks with different polish methods and split a lot of new brass to load/shoot to failure and see it if there's a measured difference but even then it would only be a small sample size and I'd bet there wouldn't be a significant difference.......

 

[bohem]

We put a light cross hatch on the chambers before they go out the door. Here's background on why:

There are rumors/myths about how an over-polished chamber won't let the brass grip the walls and bolt thrust will be too high. BUNK. I see people spouting this and remind them that we're talking about a piece of brass tubing that's 0.012-0.016" thick and somehow that is going to functionally aid in retaining the 8,800-21,000lb of bolt thrust on the locking lugs. No way. I have a relatively unique background in the gunsmithing world and when stuff like this comes up my first reaction is to sit down and solve the engineering problem before going with the flow that says I need to do something a specific way. This is a 2nd year engineering school application, not high level stuff.

Water in the chamber isnt' causing problems because of reduced friction. It's causing problems because water is incompressible and the fluid is taking up the air space that allows the brass to expand a little bit and effectively reducing the pressure volume for the powder to work in, this spikes pressure. I don't have a problem with water on my ammo at a match with a very simple solution. It involves not loading to 70ksi or more, I keep loads to 55ksi and my brass lasts a LONG time, ammo doesn't act differently in hot or cold weather, it's reliable and consistent.

It still gives me a little bit of irritation that we have to put a light cross hatch in the chamber before taking it out of the lathe. The finish that comes from a carbide reamer backed by an 11,000 lb CNC turning center running specialized, enrichened coolant through the bore at 300psi is damn near a mirror to start. Even 1200 grit wet paper degrades that surface finish, but we do it so that the phone doesn't ring with complaints from folks who only know what the internet has told them. At the point that phone call happens you are always the bad guy not standing up to take responsibility for your 'bad work' . Doesn't matter if the customer is trying to convince you the sky is purple cuz some unqualified statement on a gun forum said so, you're wrong. A similar situation happened last week with a guy who has 5 of our barrels. The latest one showed up a feature on the polishing of the contour that we have used for 5 years. He never noticed it before so now he's mad that we do it that way, someone from another shop told him we did it wrong and now he wants a $200 refund and doesn't want to send it back to let me re-polish the exterior. So I try to avoid that phone call by putting a light cross hatch in the chamber.

I suspect that the "polishing" myths are related to the need of less equipped or less adept gunsmiths who were cutting chambers that looked awful but they needed some sort of excuse to give the customer who was complaining about the work quality. Unfortunately, just like the "shoot 1, clean thoroughly for 50 shots" bunk this myth also stuck even when the email discussion admitting it was a marketing ploy came out after the gunsmith who started it passed away.

Titanium is a mediocre material for a receiver. Roughing up the chamber isn't going to solve lug setback failures in a Ti action, using a steel breech insert ring will do that. Hertzian Contact pressure is the failure criteria to look at there, find a way to stop the contact stresses from exceeding the compressive failure limit of the material. You either need a material with a higher compressive yield or more of the same material with an effective way to spread the load over more area. Since the first option is steel and the second option doesn't fit in the action then a hybrid approach works great... put a steel ring in there to spread the load out into more of the Ti. Kelbly's have been doing it for decades with their BR actions made from Aluminum.

 

[Hellbender]

Bohem, I always wondered about the water problem, and your explanation makes more sense than reduced friction on the case walls, as even in the quoted engineering study, the slickest (oiled case/polished chamber) vs. a dry/320 finish only increased bolt load by 1000-1200 lbs or so.

But I also wonder if reduced case volume alone would cause what is seen, I might play with Quickload a little and see how much it actually affects chamber pressure.

I also wondered if when the case expanded, it somehow pushed the water forward into the bore and caused a slight obstruction, but I always thought (scary) the gases escaping in front of the bullet would clear out what little would be there.

Probably some of all 3, I would guess.

I also load to lower pressures and have never experienced the problem, BUT, I have seen it with factory loads, specifically 5.56, 6.5 Creedmore Hornady loads, and military 7.62x51.....all possibly loaded to higher than "normal" 55k pressures.

Thanks for your insight!

 

[Ledzep]

It's not case volume, it's chamber volume. If the water keeps the case from expanding, it's similar to when you have too much neck wall thickness (neck-turn chamber with non-turned brass, size/trim down a .30-06 case into a .308, etc... for a slip fit into the neck area)... Pressures spike incredibly fast-- like starting book loads will still blow primers.

Water down the bore is no better. Usually it causes a hydraulic ring bulge where ever the drop of water was in the bore... in the best-case scenario...

 

[LRI]

The last 3 rifle's I chambered, both with SS and CM barrels, were left raw as the finish reamer left them. I have yet to encounter a problem with this method.

I have been reading some on the topic of polishing chambers. There seems to be 2 schools of thought about the topic. To be clear this is not discussing trying to eliminate dimensional problems such as pitting, scoring, etc. This is how a new chamber is finished, primarily for a bolt action rifle.

1. Leave it alone. This maintains the tightest dimensions. Break the sharp corners at the entrance, but no internal polishing is done to the chamber wall.

2. Polish it to varying levels ranging from very light to mirror polish. Those of a benchrest persuasion tend to favor a high polish. Others a light polish to break the smooth surface left from reaming.

The main discussion surrounding this topic is the relationship between the brass case and the chamber, specifically how well they grip each other during firing. It is proposed a medium or course polish leaves a rough surface for the brass to grip. This in turn reduces bolt thrust which is safer and improved action longevity. Alternatively, its argued a mirror polish has more grip, is more attractive, improves brass life and so on. There is clearly a broad range here

I have never polished one. I am considering these philosophies but dont have any experience polishing. Im inclined to try a medium scotch bright IF I decide to try anything.

My application is hunting guns, match and target guns.

What do you do and why? What can you share, suggest, etc.

Thanks

What I've done:

First, if the tooling is made correctly, in good shape, the setup is correct (to include work holding), and your lathe does not have any underlying problems (tail stock alignment, way slop, spindle bearing runout, etc. . .) then the finish should come "off the tool" pretty darn good to start.

"Polishing" is a term that unfortunately doesn't say a whole lot. We can go down this rabbit hole for days, but if your having to start with a "shingle" to remove tooling inclusions, then there's issues that need to be solved first.

Assuming the tool does a good job and your finish as machined is to standard, the path I've taken is to "hone" the chamber with 320 at 1000 rpm to create a cross hatched finish similar to a fresh cylinder bore. 60*X30* pattern. I follow up with green scotchbrite. The scotchbrite is merely to knock off any raised edges (were talking tenths of a micron here) created by honing with emery. I got this from when I was a maggot in race engine shops up/down southern CA. Flexhone was all the rage back then as it knocked off the edges left over from cylinder wall honing.

My attempt is to create a smooth finish that has a microscopic "tooth" so that when the brass balloons to fill the chamber, it becomes "bitten" by the chamber walls. This is merely an attempt to reduce the amount of brass flow towards the neck. The idea being if its stuck, it won't rob as much material from the web after each fire/resize. If its too rough, the brass will have a frosty look after firing. Too smooth and its not all that different from when it went in. A satin appearance is what I shoot for.

Whether or not it works is more or less a guess. It was told to me a couple decades ago, it made sense (to me at the time anyway) certainly doesn't hurt anything, and it creates a nice, uniform finish when done correctly.

Hope this helps.

C.

 

[Rprecision]

What I've done:

First, if the tooling is made correctly, in good shape, the setup is correct (to include work holding), and your lathe does not have any underlying problems (tail stock alignment, way slop, spindle bearing runout, etc. . .) then the finish should come "off the tool" pretty darn good to start.

"Polishing" is a term that unfortunately doesn't say a whole lot. We can go down this rabbit hole for days, but if your having to start with a "shingle" to remove tooling inclusions, then there's issues that need to be solved first.

Assuming the tool does a good job and your finish as machined is to standard, the path I've taken is to "hone" the chamber with 320 at 1000 rpm to create a cross hatched finish similar to a fresh cylinder bore. 60*X30* pattern. I follow up with green scotchbrite. The scotchbrite is merely to knock off any raised edges (were talking tenths of a micron here) created by honing with emery. I got this from when I was a maggot in race engine shops up/down southern CA. Flexhone was all the rage back then as it knocked off the edges left over from cylinder wall honing.

My attempt is to create a smooth finish that has a microscopic "tooth" so that when the brass balloons to fill the chamber, it becomes "bitten" by the chamber walls. This is merely an attempt to reduce the amount of brass flow towards the neck. The idea being if its stuck, it won't rob as much material from the web after each fire/resize. If its too rough, the brass will have a frosty look after firing. Too smooth and its not all that different from when it went in. A satin appearance is what I shoot for.

Whether or not it works is more or less a guess. It was told to me a couple decades ago, it made sense (to me at the time anyway) certainly doesn't hurt anything, and it creates a nice, uniform finish when done correctly.

Hope this helps.

C.

Thanks for your insight. My chambers have always had a good finish with no imperfections right off the reamer. I like the idea of honing as you describe. Thanks for your time!

 

[Rprecision]

We put a light cross hatch on the chambers before they go out the door. Here's background on why:

There are rumors/myths about how an over-polished chamber won't let the brass grip the walls and bolt thrust will be too high. BUNK. I see people spouting this and remind them that we're talking about a piece of brass tubing that's 0.012-0.016" thick and somehow that is going to functionally aid in retaining the 8,800-21,000lb of bolt thrust on the locking lugs. No way. I have a relatively unique background in the gunsmithing world and when stuff like this comes up my first reaction is to sit down and solve the engineering problem before going with the flow that says I need to do something a specific way. This is a 2nd year engineering school application, not high level stuff.

Water in the chamber isnt' causing problems because of reduced friction. It's causing problems because water is incompressible and the fluid is taking up the air space that allows the brass to expand a little bit and effectively reducing the pressure volume for the powder to work in, this spikes pressure. I don't have a problem with water on my ammo at a match with a very simple solution. It involves not loading to 70ksi or more, I keep loads to 55ksi and my brass lasts a LONG time, ammo doesn't act differently in hot or cold weather, it's reliable and consistent.

It still gives me a little bit of irritation that we have to put a light cross hatch in the chamber before taking it out of the lathe. The finish that comes from a carbide reamer backed by an 11,000 lb CNC turning center running specialized, enrichened coolant through the bore at 300psi is damn near a mirror to start. Even 1200 grit wet paper degrades that surface finish, but we do it so that the phone doesn't ring with complaints from folks who only know what the internet has told them. At the point that phone call happens you are always the bad guy not standing up to take responsibility for your 'bad work' . Doesn't matter if the customer is trying to convince you the sky is purple cuz some unqualified statement on a gun forum said so, you're wrong. A similar situation happened last week with a guy who has 5 of our barrels. The latest one showed up a feature on the polishing of the contour that we have used for 5 years. He never noticed it before so now he's mad that we do it that way, someone from another shop told him we did it wrong and now he wants a $200 refund and doesn't want to send it back to let me re-polish the exterior. So I try to avoid that phone call by putting a light cross hatch in the chamber.

I suspect that the "polishing" myths are related to the need of less equipped or less adept gunsmiths who were cutting chambers that looked awful but they needed some sort of excuse to give the customer who was complaining about the work quality. Unfortunately, just like the "shoot 1, clean thoroughly for 50 shots" bunk this myth also stuck even when the email discussion admitting it was a marketing ploy came out after the gunsmith who started it passed away.

Titanium is a mediocre material for a receiver. Roughing up the chamber isn't going to solve lug setback failures in a Ti action, using a steel breech insert ring will do that. Hertzian Contact pressure is the failure criteria to look at there, find a way to stop the contact stresses from exceeding the compressive failure limit of the material. You either need a material with a higher compressive yield or more of the same material with an effective way to spread the load over more area. Since the first option is steel and the second option doesn't fit in the action then a hybrid approach works great... put a steel ring in there to spread the load out into more of the Ti. Kelbly's have been doing it for decades with their BR actions made from Aluminum.

Thanks for the information. I respect your position on this and can say I share many of your views.

I have always been of the opinion if everything is to proper dimension surface finish (within reason) bears no role in function.

Thanks

 

[mram10]

Good read about an elaborate test regarding chamber finish

Rifle Chamber Finish & Friction Effects on Bolt Load and Case Head Thinning Calculations done with LS-DYNA

 

[Ledzep]

Post a picture of a fired case if you can. Depending on how bad the ring is below the shoulder it might not be worth the time/effort messing with it vs. letting Ruger fix it.