Home Forums Hunting & Shooting Gunsmithing Bolt thrust and chamber finish

I've been told by a couple different experienced gunsmiths that a rifle chamber shouldn't be polished too finely because it could cause excessive bolt thrust. I've never experienced it before, but think I am now, and thought I'd get some opinions on it.

The cartridge itself doesn't really matter for this discussion, but it's a wildcat cartridge for the AR15 platform using 6.8 SPC brass. The barrel came from one of the boutique companies specializing in these things, and it is Melonite treated, with a high mirror polish on the chamber.

Using the manufacturers published loads for this barrel, I'm not seeing pressure signs in the primers, and velocity is about the same as they list, but I get serious brass flow into the ejector and extractor cutouts in the bolt. The usual suspects like headspace, bolt timing, and tight necks have been ruled out; I think I've encountered a case of high bolt thrust due to a slick chamber.

Has anyone else here run into a similar situation?

Your pressure is north of 70k psi.

No, it's not. This isn't a high-pressure situation; I am specifically asking about a chamber that is polished too slick.

I don't polish after reaming, I could see of higher bolt thrust from a mirrow chamber. I never had a problem with just reaming. I have a friend that will run 320 grit wet sand paper with oil after chambering. Even if he uses my reamer he can't use my loads without flowing brass into the ejector hole. He uses Hart barrels I use Shilen barrels.

If I oil a case, I see no evidence of increased bolt thrust.
In tests using a Lee Enfield, dry cases would separate; oiled cases would move the shoulder forward until the cartridge could no longer be chambered. In a Remington action, nothing happened either way
In a strong, front locking action, even with the case lubricated, compression of the bolt head is less than .001".
I made up a fixture to hold a piece of brass with about 1/2 inch exposed. I could push hard enough, on a piece of rod with a T-handle to deflect the head by .0015.
Most full length sizing dies are pretty shiny but a case will stick in one pretty well. I doubt if the pressure require to size a piece of brass is equal to 60,000 psi yet the brass sticks nicely in that shiny, hardened, chamber.
I'm willing to bet that, if you polish a chamber to a mirror finish, and fire a cartridge with too much head clearance, you will see a head separation which would not happen if the case didn't grip the chamber wall.
Until I see solid, scientific evidence to the contrary. I'll continue to believe it ain't a problem. If a well polished chamber will grip a case well enough for it to separate (and it does) bolt thrust is not an issue. GD

Someplace on the web someone did a study.

Try 6mmBr, or one of the other accuracy forums. I think he even proved that a near mirror finish to be better. So, play with your search engines.

It might have even been in the old Precision Shooting magazine, and not on the web.

I have not near the experience of Graydog, but I stand with his opinion on this issue.

Aren't those two sentences contradictory, or am I reading it wrong? The second sentence sure sounds like evidence of increased bolt thrust, by allowing the case to fill the chamber instead of separating from excess headspace.

Swarf, good suggestion, I do have a bunch of the Precision Shooting magazines and will look through them for an article on this.

One more note about what I'm seeing - significant brass flow into the bolt cutouts, but rounded primers and velocity that matches the manufacturer's data (duplicating their loads). The brass flow is evident even at slightly reduced loads.

Using the same brass, primers, and bolt in another chamber I get flat primers before any signs show up on the brass.

Not arguing just trying to learn. Seams to me that bolt thrust will be the same weather the entire case moves back.Or just the head do to stretching at the web juncture.

It's on Varmit Al's web site.

Chamber Finish/Bolt Thrust

Yondering,
Not contradictory at all. In both cases, bolt thrust was significant. When dry, the case separates ahead of the web. If oiled, the case moves back and the shoulder moves ahead. Only the springiness of the Lee Enfield makes this obsevable. If the chamber is polished real well but the case is dry, the case still separates.
I did a lot of testing using a Lee Enfield to try and get a feel for the advantage of the "improved" case for reducing bolt thrust but could not see any evidence for that either.
Grip is grip. Once the case grips the chamber walls sufficiently to separate the case, gripping it more means nothing. The only thing which seems to eliminate adhesion to the chamber wall is lubrication.
I think your problem may well be a gas port which is too large or a powder which is too slow and is creating too much pressure at the port so that the breech is starting to open prematurely. One often sees the same sort of thing on Remington 742's. GD

No didn't happen to me but when my GS reemed a chamber in a new barrel for me, He told me about what work and why he didn't polish the chamber to much. So he was aware of this possible situation and would not do to much polishing.

AJ, thanks for the link.

greydog, that makes sense with the additional info, thanks. This is one of those things though, I don't think you can prove it doesn't happen, but certainly not all cartridges and chambers would do it. I do follow your line of reasoning though, and have never encountered an instance before that led me to believe the chamber finish was an issue, until now.
FWIW, as I mentioned in the first post, bolt timing was eliminated as a possibility; it still does it with the gas port blocked off.

Ray and Gemby, that's the sort of thing I've heard before, but hadn't encountered personally. On my own stuff, when I ream a chamber I follow up by burnishing with steel wool wrapped on a brush, but that's all. The other barrel I mentioned that works fine with the same brass, primers, and bolt is one of mine done that way, although they are different calibers so not a direct comparison.

A lite polish is all you need if any, never a high polish. lite polish will just show any scratches on the chamber wall.

Also another point, if anyone missed it, the chamber in question has a Melonite QPQ finish, which makes it slicker than just polished bare steel.

I'm thinking that makes it more prone to any issues that an over-polished chamber might have, if any. But I haven't used many other Melonite/nitride treated rifle barrels either.

The only times I've seen brass flow is either due to running high pressure, or using soft brass. High back thrust can cause damage to the locking lugs of a bolt and or the mating lugs on the action, but IMHO back thrust is a completely separate issue than brass flow.

I would suggest trying a different brand of parent brass to see if the issue persists. If it does, I'd look at backing down your loads.

Force against the bolt face will be equal to PSI X case head area either way. Force against the bolt face may be less if the case separates, and some of the pressure is vented.

The brass case acts as a plug to seal chamber gases. If the chamber is rough 'not highly polished' effectively increases the surface area of the chamber. Many 'smiths' will run a little scotch brite in the chamber as a final finish after reaming. Also, running 000 steel wool on the case doesn't hurt either.

this

Check the diameter of unfired vs fired brass.

Is the chamber oversized...causing the brass flow?