The set back pressure problem in my experisnce works as discussed in this post for rifles. Setting back the projectile reduces the leading edge of the pressure curve.
The math on this, (all other things being equal) as far as I know it is controlled by the expansion ratio. Case volume + bore/freebore volume divided by case volume.
1. In micro seconds the firing pin strikes, the primer backs out of the case, pressure builds, the projectile launches from the case and lodges itself in the bore.
2. At this point going forward the exact same volumes and parameters are applied to the two scenarios. A properly seated projectile and a projectile seated more deeply into the case.
The difference introduced by seating the projectile deeper in the case reduces the case volume and increases the bore/freebore volume.
So the actions that take place from ignition to the bullet lodging into the bore are only different in reaction because the initial area from the front edge of the projectile to the bore is larger.
This while not necessarily reducing peak pressure, must produce a slightly less vertical leading edge to the pressure curve.
An overcharged case depending on how highly over charged it is will still produce an over charged peak pressure. The increased free bore may be enough to circumvent this over charge peak pressure if it is a very slightly over charged load.
3. Going forward from paragraph 1, with the lodged projectile in the bore the powder continues to burn which causes the pressure to build, reseating the primer, expanding the brass of the cartridge sealing the rear of the chamber and allowing the pressure curve to build to peak as the projectile travels the distance of the bore and then the pressure recedes.
The math on this, (all other things being equal) as far as I know it is controlled by the expansion ratio. Case volume + bore/freebore volume divided by case volume.
1. In micro seconds the firing pin strikes, the primer backs out of the case, pressure builds, the projectile launches from the case and lodges itself in the bore.
2. At this point going forward the exact same volumes and parameters are applied to the two scenarios. A properly seated projectile and a projectile seated more deeply into the case.
The difference introduced by seating the projectile deeper in the case reduces the case volume and increases the bore/freebore volume.
So the actions that take place from ignition to the bullet lodging into the bore are only different in reaction because the initial area from the front edge of the projectile to the bore is larger.
This while not necessarily reducing peak pressure, must produce a slightly less vertical leading edge to the pressure curve.
An overcharged case depending on how highly over charged it is will still produce an over charged peak pressure. The increased free bore may be enough to circumvent this over charge peak pressure if it is a very slightly over charged load.
3. Going forward from paragraph 1, with the lodged projectile in the bore the powder continues to burn which causes the pressure to build, reseating the primer, expanding the brass of the cartridge sealing the rear of the chamber and allowing the pressure curve to build to peak as the projectile travels the distance of the bore and then the pressure recedes.
Last edited: