This happened to me and my riflesmith confirmed it.
He knew of it happening with a Pattern 17 Enfield
also in New Mexico. It involves the GCheck dislodging
and falling on top of the powder charge during the seating or later. Even with "crimped" supposed Gchecks.
My pre-64 M-70 in 338-06 developed probably close to 75-80K PSI and I had to replace the extractor. I was lucky that no more damage was done to the rifle.
With straight wall cases or the 30-40 Krag-the neck is long enough to keep the GC in the neck area rather than over powder in the case. It CAN happen and DID happen to me.It will not happen again. I have been handloading carefully for 50 years.
This has NOT happened again because I have found that GCs are not needed on my Sharps or 1885 HiWall rifles. Or anything else that is bottlenecked. There are safer ways for me to shoot economically.

Not trying to convince anyone. There will be doubters. Don't need to prove it-Been there and so has my gunsmith with others. Not all venomous snakes rattle, and I don't go to Las Vegas or the range to gamble.Just tellin the tale.

I am not arguing, but would love to understand the reason for the increase in pressure.

I would have assumed that the gas check would just get blown out the muzzle like a wad?

Interesting.

But how do you know it happened that way.

The gas check fell off. So what? We are going to need some sort of semi scientific explanation for how a loose gas check contributed to high pressure. It was smaller than the powder area so could not cause an obstruction. Trying to envision some sort of obstruction scenario...... Not that I don't believe your gunsmith said it; I just don't believe the theory. Didn't say you were a careless handloader so don't go there.
As for BP cartridges, those do not need gas checks anyway.

I have doubts that the check dislodged and caused an obstruction.
Tell us about the bullet you are using.
Did you seat the check and lube at the same time?

What you describe leads me to think that, if the bottom one or two grooves of the bullet are below the neck, and lubed, you more likely had a problem with powder clumping around the bottom of the bullet.
Best practice is to NOT have any lube on any portion of your bullet that will be below the neck.

I also frequent a cast bullet site and have for about 20 years.
There has never been a single report of high pressure caused by a gas check.
Unless you have better evidence your gunsmith is pumping sunshine up your dress.

The sky is falling!

Exactly how did your "riflesmith" determine/ confirm this ?

You don't have to convince anyone, I don't think there would be a single person convinced. I would be amongst the shooters around the world who a collectively fire millions of cast bullets a year and have not heard of a single proved case where a gas check has caused a pressure issue.

Nobody can say a gas check has come off the base of a bullet in a loaded cartridge as you cannot see inside the cartridge and if you pulled a cast bullet to look you could expect to see in some cases the pulled bullet has left the gas check behind, proves absolutely nothing in respect of a gas check falling off into the powder of a loaded round.

Gas checks are relatively soft copper, or often aluminium if hand made, so would not form any great resistance to being shot out of the case behind the bullet if they had fallen off the bullet base.

I would be suspecting the powder charge or load characteristics if pressure was high enough to blow extractors. Low charges of slow burning powder under some conditions can cause pressure spikes or even detonation as has been seen in some instances over the years. I myself have experienced slow fires and hang fires, some times a precursor to detonation, with certain loads and cartridges when loading with both jacketed and cast bullets over the years.

Your gunsmith is purely guessing as to the cause of your pressure spike.

I agree. I shoot cast w/ GC all the time with nary an issue. Even a dislodged GC would not cause you a problem.

Obviously a problem occurred.

But blaming it on a gas check is net very easy to believe.

A gas check is smaller than the neck of the case in its largest size, so how would that cause this problem?

I think the problem lies somewhere else, and to avoid it happening again, you might wish to find what it was.

quote:

Originally posted by Saeed:
...to avoid it happening again, you might wish to find what it was.

Sage advice