Peak pressure is not attained immediatly upon fireing. It takes time for the bullet to move down the barrel before Peak is attained.
Proof of that is simple. If Peak pressure is instant then a 2" barrel would develop max FPS in any rifle.
A bullet shoved back in the case pretty much only only affects initial pressure, once it's moving, powder burn rate increases to Peak pressure & should be roughly the same in either round (regular round or one with set-back bullet).
A heavily crimped bullet certainly increases initial pressure & can be a dangerous situation if the round was developed to Max with-out a crimp, then a heavy crimp added.
In other words a neck loose enough to permit a bullet to slide around in it certainly will decrease initial pressure, enough to balance out the extra pressure of a bullet set back into the case(less volume).
Perhaps I should have made it more clear in other post in talking about pressure but I would have assumed most would realize we were talking about initial pressure build-up when talking a set back bullet.
Peak pressure in a 30-30 is attained by the time the bullet has moved about 3/4". Magnum cartridges with slow powder reach peak pressure at about 2" and handguns with small cases and fast powder reach peak pressure in 1/8" to 1/4" of travel. (yep, the bullet hasn't even left the case)
Proof of that is simple. If Peak pressure is instant then a 2" barrel would develop max FPS in any rifle.
Sorry, but that is not quite right. Don't confuse
velocity with
peak pressure and the
pressure curve within the barrel. The "factors" you mention are minor. A heavy crimp will slightly advance the timing of peak pressure, but not have a significant effect on the peak.
I am very familiar with internal ballistics and the computer modelling of it. It is very complex and every time you change one variable, everything changes, and it is difficult to explain in a couple sentences. However some variables are much less significant than others in regards to PEAK PRESSURE. A loose neck will change the timing of the peak, but will not have much of an effect on it. However a large PERCENTAGE change in case capacity has a huge effect on peak pressure.
So a small volume, straight walled case will have a much higher % change for a small setback of the bullet (such as 9mm), so a small setback will have a major effect on pressure. But a large case with a small diameter bullet, (large overbore magnums) will have a very small change in case volume for a small setback, and the peak pressure increase will be very minor.
It is very difficult to fully understand what happens with internal ballistics because EVERYTHING is interrelated and you can't change one thing without changing everything. Simplifications and generalizations are fine and don't really affect the outcome (velocity) too much when changes are small, but that is not what is actually occurring. For larger changes, then all the simplifications break down really quickly.
