I necked some .308 Lake City brass down to .243 and did not turn the necks. Loaded with 40 gr IMR4895 and 100 gr bullets, this should have been about 60 kpsi and 3050 fps. The three rounds loaded the same, did not produce the same pressure signs on the brass. The neck was barely too tight on one, and too tight on the last. The 1938 Turkish Mauser with Adams and Bennet bull barrel gave my face a breath of stinky air that bounced off the bench on the last one.

This is counter intuitive that there is so much change in pressure with so little change in neck squeeze. Just like it is counter intuitive that oversized bullets cause no increase in peak pressure. To understand both, I have to think in the time domain, where the bullet swaging increase in pressure does not overlap the powder burning peak in pressure, while the pinched bullet causes a delay which reduces the burn volume at peak pressure.

If I understand what you wrote, it's not the case neck tension on the bullet that caused the pressure spike. It was the thick case neck. There was no room for the case neck to expand as the powder burned and the gas volume increased.

You're lucky.

George

George is correct. When you fire a round off, even in a tight neck chamber, there is a litle slack that allows the neck to be pushed back enough for the bullet to easily leave the brass. If you have the jam the round into the chamber you just took all the slack away and your brass was unable to release the bullet until it built up enough pressure.

I think that bullet pinch can be a problem with 9x19mm as well.
I was working up a Power Pistol overload with 158 gr XTP 1.169", and it was ok with 8 gr, but the primer always fell out at 8.5 gr.

Because the powder was being severely compressed, I decided to correct for bullet pinch.
I double compressed the powder charge [to prevent bullet squish] and resized the loaded ammo with a 9mm sizing die with the decapping stem removed.

I was then able to load up to 11 gr, and there we no more pressure problems. This is stout load as the Alliant max load for 357 mag is 8 gr and has a 1.59" OAL.

I am concluding that bullet pinch was what caused the pressure problems at 8.5 gr.