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?

I was taught to polish a newly cut chamber with first wet 320 and then wet 400 black wet/dry paper.

The British used to use heavily lubricated service rounds as proof rounds.

+1



Yes the case ( if supported well enough by the chamber and blot, and within its limits) will balloon, and seal everything off. The pressure force acts in all directions. If the area of the chamber is increased more force, PSI X area, acts against it, (the chamber). Still the back thrust against the bolt is PSI X case head area. Or maybe area of the bolt face it self, if the case doesn't seal or ruptures.
I'm really not trying to argue with you that know what you are talking about. I have followed this being disused before. I am curious about it. And trying to get it explained to me. Thanks.

PSI* internal area of the case...

You are saying that the case walls at the stress ring will
hold back the chamber pressure.
A case will grab and hold until something like 30/40 KPSI,
Then it will come back against the breech.
An oiled case will come back and expand forward avoiding
the stress ring.
Once the case comes back it will exert the same force oiled or dry.
Oiled were used by the old timers to check the locking
on low pressure loadings.
Ackley claimed to fire a 94 Winchester without locking lugs.
If he had ran the pressure up it would be a different outcome.

`I am saying that it is the bolt face that supports the case head, and the chamber supports the case walls. Making it possible for the case to hold the pressure. Just like the tire and wheel support the inner tube.The stretching at the stress ring occurs because the case walls grip the chamber. But the case head is pushed back against the bolt face. The pressure is inside the case and it holds it at all points, until it ruptures and or ruptures and separates. (Think head space).
In a modern high intensity cartridge the case head comes back against the breach anyway. The walls grip the chamber. That is what causes the stress ring. With the oiled case the entire case comes back together. Avoiding the stress ring. The thrust against the breach is still the same. Unless the inertia picked up by the case moving back contributes, which I doubt. Or the straining of the brass at the stress ring contributes, which I also doubt.

If that is true. Then the case (walls griping the chamber, and head supported only by the inertia of the bolt and linkage). Held the pressure.

Precisely. Most of the case. From what you call the stress ring forward would likely remand in the chamber. The case head, bolt, and other 94 parts. Are what the doctor or mortician would have removed from P.O.s body.

Actually that can be significant. For an extreme example, consider a 40 S&W round fired in a 10mm chamber, when the round is in front of the extractor. This does not create a high pressure situation, but impact of the brass against the breech face causes severe damage that usually would be considered "high pressure signs".

Also note the missing piece of case rim; this wasn't ripped off during extraction, is was pushed off towards the case mouth as the case moved towards the breech face during firing.

Maybe the cartridge does matter!


How much taper does that wildcat have base to shoulder?

Ok. Cool. I didn't realize that. Thanks!

From what you're describing, it sounds like a situation of excess pressure. I've not played with meloniting, or other forms of salt bath nitriding, but I'm wondering if that has something to do with what you're seeing too.

I shared my thoughts on bolt thrust a long while ago here;
http://www.24hourcampfire.com/ubbthreads/ubbthreads.php/topics/997867/5

There's some interesting bits in there to help you connect the dots.

Good luck.

What would it be called back in the days when we were chambering Remington 700's to 30-378 Wby and found out after like 200 rounds the heads space was changing due to action streching. If the case grabs the chamber and there is no bolt trust what was causing the action to streching

...too much of a good thing. Or stuffing 10 lbs of flour into a 5 lb bag.

Not much taper at all. It's a 30 Herrett wildcat in an AR15, basically an AI format, with less body taper than the parent 6.8 case.

What does the chronograph say? I once chambered a 375h&h and had no brass to test the rifle. I used a few 300 wby cases to form the 375 brass. Dimensions looked right.....but the Web was different. First round stuck the bolt. I backed off south of the book and did it again. The lights said it was fast.....and they were right.

What brass did you use to form your Rimless Herrett cases? From our HAGAR experiences, some 30 Rem and 25 Rem brass is soft and not suitable for higher pressures.

Even the SPC brass can be spotty. SSA seems to have put out some bad (soft) lots. Hope Nosler cleans up their QC.

Thanks for posting that link ChrisF. Great reading.

Brass is all 6.8 SPC - Nosler, S&B, and two batches of Hornady. The S&B and one Hornady batch are pulled from factory 6.8 ammo. All do it, although Nosler seems the worst; it does weigh about 10 grains more so capacity is slightly reduced.

The velocity readings are right in line with what the manufacturer says for the powder charge, and primers are well rounded too. The only thing wrong is the brass flow, at ~2gr and ~200 fps below what the manufacturer says is max.

Just checking off possibilities, so please pardon the obvious question; No residual oil on cases from fireforming?

greydog's overgassing possibility is the realm of Rost. I'll point him over here. (I just realized that statement is true in multiple ways!) Are you able to remove the gastube easily and refire safely?

Other than that I'm stuck. Please keep updating.

Chris got my attention, its a bad month at work, fire prevention week etc...

But a quick glance, IMHO I"d shoot the gun wihtout a gas tube in place and see what the results are.

It simply could be too much gas starting to unlock the action early, allowing brass flow.

It also may be something otehr... but the indication of normal primer would lead me to think gas flow as Chris noted.

The easiest way is simply remove the tube, and then cycle action by hand. Problem goes away, you have an answer. Problem stays then at least you rule out an oversized gas port for the system(easy peasy way to make things cycle in an AR when we are in too much of a hurry to calculate things and drill the right size gas port.

Guys, thanks for the comments, but I've answered half the questions in this thread already in my first couple posts. Opinions or suggestions from people who didn't read all the details aren't as helpful as they might think.

Shutting off the gas was one of the first things I tried, by installing a spare gas block backwards. Brass flow still happened of course; timing can cause swipes but it doesn't cause the case head to push back into the bolt cutouts.

Sorry guys, that came across a lot ruder than I intended, and I apologize. It wasn't directed to anyone in particular, but just a general frustration about how this forum seems to operate most of the time.

ChrisF, no oil on the cases or the chamber. A similar question along those lines got me thinking about slickness of the chamber though; if oil on the cases can cause an issue, maybe a slick chamber finish can too. Some experienced gunsmiths seem to agree.

320 then 400 is as fine as a chamber should be. Finer is the same as lube, no grip.

That's what I've been told, and this barrel makes me agree. Have you had personal experience with a chamber that's too fine, and if so, what did it do?

If the chamber is plated or coated with a finish which provides some lubricity (not sure that's a real word), the case will certainly fail to grip the chamber walls, just as if it were oiled. I do not think this truly affects bolt thrust, however, unless deflection of the bolt is sufficient to stretch the brass.
For those who think a high polish is just the same as lube, make a sizing die of 416 stainless and polish it to a mirror finish inside. Shove a fired case into it and see how solidly it sticks. A polished surface is nothing like a lubricant.
If the rifle and ammo have some head clearance and the chamber is lubed, whether with oil or a coating, the case head will be deformed because it contacts the breech with some velocity. This is not increased bolt thrust but the deformation of the case head is the result of energy transfer due to the velocity at which the case moves back. GD

Once again, what does the chronograph say? If you are magically getting a free 200fps......

People state on here that there no bolt thust as the case grips the chamber wall. If that is the case why did TC never chamber there contender for cartridge like 243, 308, 7mm08, ect, they had to use a stronger encode frame. I know from experience when chambering a 700 action for 30-378 and 338-378 the action went out of head space after 200 to 300 rounds

I don't think anyone is saying that.

Oh ok then what are they saying you can't tell me that if you polish the chamber wall smooth as a piece of glass that is
t doesn't increase bolt thrust

I started the thread asking if anyone had first hand experience (not theories!) of increased bolt thrust with a polished or slick chamber. I don't think anyone was suggesting there could be a situation with no bolt thrust at all.

Got ya. I don't polish chambers not to cause bolt thrust

Sorry, I'm having trouble following the double negatives in your sentences; not sure what you mean by that last one.

Do you mean that you don't polish chambers, because you don't want to cause increased bolt thrust?

I don't polish

This may be the Experiment with chamber finish you refer to.

http://varmintal.com/a243z.htm

Very few perhaps no one may have experience with your exact circumstances. Polished Melonite chamber finish really reduces the number of folks with same experience. I would buy into the theory that the polished Melonite finish will simulate a lubricated bore. Friction coefficient changes significantly with the polished SBN treated steel as I understand the process.

Any Idea How much head space you are starting off with b4 fire forming the brass in question? Or are you experiencing the same flow issues with fire formed & neck sized brass.
I am no expert on the situation at hand yet I fear you really are pushing the pressure limits in this case . I will be interested in seeing what more is learned with this delima.

Stay Safe!

Thanks for the link.

Headspace was .002"-.004", although I also tried partial sizing (not bumping the shoulder) and crush fit on the fireformed brass.

The only thing that made the brass flow go away was firing the cases 5+ times at lower loads, then they would handle the manufacturers loads with no brass flow. When I sent it back to the manufacturer, he tested the barrel with his own brass (which had been fired multiple times too) and proclaimed it OK.

Seems to me the brass work hardening masked any issues with the barrel, but he disagreed.

Doesn't matter now, when I got the barrel back I found he'd "polished" the chamber and somehow left a sharp lip in the neck, cutting through the melonite on one side. Sent it back a second time and was finally able to get a refund.

I'll be making a new barrel for myself in another version of this same wildcat, but with the shoulder bumped forward a little. No melonite treatment or slick polished chamber this time though, we'll see how it works.