Rank Amateur
Gold $$ Contributor
Very interesting thread. I think one challenge we face to understanding is comprehension of the velocities involved and the “quickness” of pressure effects. Discussion of “thin” necks above ties into the concept of gas pressure working its way between bullet and neck wall. Explosion velocity of the powder charge, and speed of the pressure front advancement, is MUCH faster than the acceleration of the bullet’s mass into the barrel. If we can conceive of the case walls blowing out to the chamber before the bullet fully leaves the mouth, and we know that neck expansion will stop the instant the pressure inside the case can escape outside the case mouth, then we know all of this is taking place in a very short period of time. The case sealing against the chamber walls is what would prevent carbon depositing all around the outside of the case. Since significant carbon is usually only found in that ideal sine wave pattern around the outside of the neck, the area below that carbon must have sealed off before gas escaped the case mouth. This effect could occur either by the gas escaping the instant the bullet base (or pressure ring) moved past the mouth, or by the peeling of the neck from the bullet by the gas “excavating” its way past the bullet as the bullet is accelerating.
Gas escapes around the bullet even after it is engraving, but there is still sufficient pressure behind the bullet and continues to build until the bullet is a short way into the lands (as @DaveTooley mentioned above) to accelerate the bullet to its muzzle velocity. So, its the initial and retained pressure behind the bullet that effects the bullet’s acceleration. The bases of my thinking on this are the relative velocities and time frames that MUST be occurring. Bullets are fast, but they are not traveling at 22,900 FPS (the speed of the gas/pressure front), especially as they start to accelerate. The entire time between powder ignition and the bullet moving past the mouth, the pressure inside the case is pressing on the brass. If that pressure can slam the thickest part of the case against the chamber wall before the gas can bypass it (after escaping the case mouth), then its ability to work through the weakest part of the system (the bullet/neck contact) and expand the weakest part of the brass (the thin neck) seems easily conceivable. Again, the bullet is moving exceedingly slowly at this point (relatively).
Gas escapes around the bullet even after it is engraving, but there is still sufficient pressure behind the bullet and continues to build until the bullet is a short way into the lands (as @DaveTooley mentioned above) to accelerate the bullet to its muzzle velocity. So, its the initial and retained pressure behind the bullet that effects the bullet’s acceleration. The bases of my thinking on this are the relative velocities and time frames that MUST be occurring. Bullets are fast, but they are not traveling at 22,900 FPS (the speed of the gas/pressure front), especially as they start to accelerate. The entire time between powder ignition and the bullet moving past the mouth, the pressure inside the case is pressing on the brass. If that pressure can slam the thickest part of the case against the chamber wall before the gas can bypass it (after escaping the case mouth), then its ability to work through the weakest part of the system (the bullet/neck contact) and expand the weakest part of the brass (the thin neck) seems easily conceivable. Again, the bullet is moving exceedingly slowly at this point (relatively).
Last edited: