Home Forums Hunting & Shooting Gunsmithing Bolt thrust and chamber finish

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.

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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.