Bullet set back

[sweet1911]

Bullet set back. I have always been a revolver person but in the last couple of years I have switched over to semi automatic. And I know not to chamber the same round more than one or two times not to get bullet setback. My question is how will bullet set back affect the performance. What can happen with a setback. I could understand a feeding issue. But what other problems could it present. And how severe wood setback have to be to present a problem

[billt]

It depends on the powder. With slow burning rifle powder it's not so much of an issue because slow burning propellants are not as much effected by it, and because the case volume is much greater than it is on small pistol cartridges. With fast burning propellants it can cause a rapid pressure spike, because the powder has a much smaller space to start burning. Pistol cartridges because of the small volume they contain, build up pressure much faster.

Add it all up and if you are running a maximum load in a small volume case with a fast burning pistol powder, and a heavy bullet, (which are more prone to setback), it can cause cause the effect of a drastic overload. This mostly occurs with semi auto pistols because of 2 reasons. One is because the cartridges in the magazine are subjected to being pushed in during recoil, rather than being pulled out like in a revolver. The second reason is because semi auto pistol cartridges like the 9 MM, .40 S&W, and .45 ACP all headspace on the mouth of the case. This means you have to use a taper crimp on them which does not hold the bullet as well as a heavy roll crimp does that can be used in revolver loads which headspace on the rim of the case itself.

This also why on some .45 ACP factory loads you will see a heavy cannurle, (sp), below the base of the bullet. This prevents the bullet from pushing back into the case. Upon firing the normal chamber pressure blows out the case to the chamber dimensions, eliminating that crimp.

[Tyler Durden]

I reload, a lot.

So on my Dillon 650, I have toolheads for 9mm .40 and .45 ACP each set up with the Lee decapping/resizing die that has also been tweaked on by Evolution Gun Works to make it an Undersize die (aka a U die).

So my loaded rounds have the coke bottle shape to them.  The crimp and the shape keep the bullet from seating any deeper.

as far as what can happen....well, if pushed short enough, it could lead to a KABOOM!