Freebore and overpressure

[Varmint Hunter]

The question in title with this thread is about overpressure.

Yes, a wet lube in the chamber is a bad thing.
It causes cases to slam back against the bolt, and then blow the shoulder forward from there. It flattens primers, and doesn't do the case webbing/head any benefit.

This has ABSOLUTELY NOTHING to do with the load 'pressure', and is not a valid pressure problem.
It didn't happen due to an overpressure condition.

I have always believed that oil in a chamber "can" cause excess pressure issues. I lightly clean my chambers with lighter fluid after a bore cleaning.

I handload for a buddy that I shoot with every week. One day he fires a handload and the bolt it TIGHT. He calls be over to his bench to show me the problem with the handload. When we manage to get the case out, I immediately notice that the case is slippery with oil. I asked him where all the oil came from and he tells me that he oiled the chamber and bore after cleaning.

We cleaned all the oil out of the chamber, and he resumed shooting the ammo without issue. That seems to confirm what I have always believed. While it doesn't seem like oil on the outside of a case could increase pressure, the results of doing so certainly leave that indication.

 

[VinceMule]

I saw my neighbor's 6.5x 55 Swede have excessive pressure with a minimum book load. The pressure was so great that the bolt was forcefully lifted up and tapped to the rear to eject the case. The case had overpressure signs. After much investigation of pulling loaded ammo, all was fine.

I looked inside the muzzle with a small flash light and the bore was gold looking with some slight black carbon. The barrel was copper plated from him shooting 160g bullets that have a tremendous amount of bearing surface.
After many applications of Montana Extreme, the copper fouling was all gone, normal pressure resumed.

 

[Mikecr]

While it doesn't seem like oil on the outside of a case could increase pressure, the results of doing so certainly leave that indication.

It doesn't increase pressure, so it's not a 'pressure' problem.
It's an oil in the chamber problem. Right?

If you oversize cases, leading to cracks/separations/flattened primers, again, that's not due to pressure.
It's due to a bad reloading plan.

If your cases won't extract due to bad bolt timing, or poor chamber/gun build, these are not pressure problems.

As each of these problems are separate and different, so is pressure itself.
If you have a pressure problem, the fix is to lower pressure. That is, fix the cause of overpressure.
You're not going to fix any of these issues with lowering of the charge, if the charge is not even the problem. Right?

 

[Varmint Hunter]

If you have a pressure problem, the fix is to lower pressure. That is, fix the cause of overpressure.
You're not going to fix any of these issues with lowering of the charge, if the charge is not even the problem. Right?

If the case is plastered to the inside of the chamber, pressure has apparently over stretched the case. In the scenario I described, eliminating the oil in the chamber eliminated the problem. If it wasn't an issue of excess pressure; how do you explain the case being so tight that the bolt could hardly be lifted?

Although I'm not inclined to test it; I wonder if the velocity increased with the oil laden case. Maybe someone else has experienced this and knows the answer.

 

[Longtine88]

Another rare cause I’ll throw out to yall is while using a quick change action be careful and always check that the barrel is screwed all the way in. Two weeks ago I was shooting next to a guy. He was trying to take his suppressor off a quick change action. Finally got it off. The set screws slipped on him and he didn’t know it or didn’t check them. He fired too shots and I could tell by the sound that gas was getting out around the case. He was lucky it didn’t do any more than what it did. He had screwed his barrel off and didn’t realize it around 0.050”!!! I was like dude always double check that when. Using a switch action. Wished I had taken a picture of his brass lol. Primers popped on the two and the brass and gas port hole was covered in carbon dust.

 

[Mikecr]

If the case is plastered to the inside of the chamber, pressure has apparently over stretched the case. In the scenario I described, eliminating the oil in the chamber eliminated the problem. If it wasn't an issue of excess pressure; how do you explain the case being so tight that the bolt could hardly be lifted?

On normal firing in a dry chamber, cases expand beginning with necks and moving back to webs.
The cases expand to sealing against and gripped by the chamber. Since this expansion is front to back, cases stretch back to the bolt face.
Ideally, with the correct head spacing, this backward expansion is within the case's elasticity. Everything is fine.

If this stretch back is in excess, a case will thin from where it last held sufficient chamber grip, and lacked grip beyond.
If the case cannot grip the chamber at all (because of a wet lube in the chamber), the entire case will slam back to the bolt face with no resistance, and then expand forward to slam against a lubed chamber shoulder. Head spacing becomes negative, and you get to knock the bolt open with a boot. This would happen at any normal pressure.

I'm confident that relatively few occurrences of difficult bolt turn/difficult extraction are caused by pressure.
It is just that pressure reveals these problems -sooner.

 

[Michael Lares]

Carbon ring can do it.

Yes on the carbon. Can do really bad stuff.

 

[cajun]

Liquids dont like to compress. I would guess oil in a chamber could cause issues.

 

[Longtine88]

Liquids dont like to compress. I would guess oil in a chamber could cause issues.

Yep. Not good. I’ve seen guys taking rifles from AC controlled rooms directly out into heat shooting. Wiping their scopes clear because of condensation then shoot……wonder if there’s condensation inside the bore and chamber?????. Guess what….pressure!

 

[Varmint Hunter]

Ideally, with the correct head spacing, this backward expansion is within the case's elasticity. Everything is fine.

If the case cannot grip the chamber at all (because of a wet lube in the chamber), the entire case will slam back to the bolt face with no resistance, and then expand forward to slam against a lubed chamber shoulder. Head spacing becomes negative, and you get to knock the bolt open with a boot. This would happen at any normal pressure.

Not looking for a disagreement but:

If the case can expand rearward (dry chamber) and stay within the case's elasticity, it would seem to reason that it could also expand forward (oily chamber) to the same extent and be within the same elasticity limits.

I do appreciate your feedback. While I've been around the block a few times, I continue to learn a few things here on LRH.

Thanks for that Len!

 

[cajun]

Not looking for a disagreement but:

If the case can expand rearward (dry chamber) and stay within the case's elasticity, it would seem to reason that it could also expand forward (oily chamber) to the same extent and be within the same elasticity limits.

I do appreciate your feedback. While I've been around the block a few times, I continue to learn a few things here on LRH.

Thanks for that Len!

If there is oil in the chamber it could prevent the shoulder from moving forward as the oil doesn't want to compress. It also takes up volume in the chamber.

 

[MagnumManiac]

@Mikecr is exactly 100% correct, with oil in the chamber, the case DOES NOT grip the chamber, the resultant back pressure forces the bolt lugs hard into the action and the case is forced forward in the chamber and it doesn’t springback normally because it is now over expanded.
Oil in the bore also raises pressure to overload levels, have seen this over the Pressure Trace twice, once with my 375 Weatherby and once with my 25-06, both incidents blew primers and locked the bolts up, but no permanent damage.

Cheers.

 

[assasinator]

CASE thrust is a primary concern in my encore. oily cases increase bolt thrust and its not good. doesnt change pressure, but can hurt my guns.

i cant use wsm, or any .532, .550 rounds. by belted mags are technically .512 even though the rim is .532.

i am getting my ar-10 rechambered to 300 RCM which has a .532 case. the ar10 can handle it.

i use 7mm PRC brass for the 300 RCM.

 

[1yotedog]

I'm back on the issue of slow powder and light bullet issue. I'm now trying to get an 8 twist Brux in 243 Win to shoot 75 gr. Hammers. Already gave up on 69 and 70 gr bullets as too short for the twist. All efforts so far have been in the 1 - 1 1/4 in @ 100 yrds. range. One test I've tried was to use IMR7955 which is slow for a 75gr bullet and a short jump (short for a copper bullet) of only 20mils to simulate the resistance of a heavier bullet. This to reduce bullet deformation from engaging the rifling too fast. The powder charge ladder moved smoothly from 3100 to 3500fps with no pressure signs evident. Accuracy was no better than Varget, Staball 6.5 and others so I've only done this once. My question is whether I was risking detonation using that slow a powder?

Edit: It was NOT IMR 7955 as originally stated but IMR4955.... is it a risk for overpressure by detonation?