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Bullet “Set Back” In Semi-Auto Pistol Cartridges

Bullet “Set Back” In Semi-Auto Pistol Cartridges
Causes, Dangers and Remedies

CHAMBERING IS VIOLENT

When semi-auto pistols chamber a cartridge, it is a violent action to the cartridge that applies undesirable forces to the cartridge i.e. the slide moves rearward at hyper velocity, bottoms out and comes forward just as fast as it moved rearward, picks up a cartridge from the magazine lips and slams that cartridge, nose first, into the feed ramp and then up into the chamber and into battery. Depending on how the bullet nose shape/profile fits with the angle and other dimensions of the feed ramp, anywhere from a great deal of damage to no damage can be done to the cartridge. The most common type of damage is what we call “bullet Set Back”. This means that because the bullet nose was slammed into the feed ramp, the bullet may have been driven more deeply into the casing……again, there is no way to measure the “Set Back” ahead of time as each make and model of pistol, will use differing feed ramp angles and dimensions and each bullet nose profile will match-up differently with each feed ramp.

SET BACK IS DANGEROUS

Bullet “Set Back” is dangerous. Most pistol cartridges have casings with very small powder capacity, so if you drive the bullet more deeply into the case, it crowds that limited powder capacity, which is the very same dynamic as adding powder, even though you did not actually add powder. So your chamber pressures will rise above what they were before that bullet was Set Back. If you are already firing a high pressure load and your bullet gets Set Back, your pressures may raise beyond what the case web or the barrel can withstand. This is not safe to the shooter or that pistol.

HAND CYCLING DOES NOT REPLICATE FIRING

Friends, Why is it that some folks insist on chambering and then rechambering and then rechambering the same cartridge over and over? If you chamber that same cartridge more than once, you should consider throwing it away. Since we are on this subject, hand chambering a cartridge to check the feeding/cycling characteristics is a waste of your time and cycling that slide by hand will not replicate the cycling and feeding characteristics of actually firing the gun……hand chambering tells you nothing except how the gun cycles any given ammo when hand chambering…….which means nothing about how the gun will cycle with that ammo when it is actually fired………never mind, you are probably ruining your ammo. Then, after hand chambering that poor cartridge multiple times and noticing it is getting shorter due to bullet Set Back, I’ll get the nasty email telling me I do not know how to make proper ammo. The situation I’ve described above is purely user error and not the fault of the ammo or the pistol.

Actually firing the pistol in question with the very ammo in question is the only way to learn how that pistol will function with that ammo—there is no other way to know.

AVOIDING BULLET SET BACK

How can we remedy bullet Set Back? Again, the potential for Set Back varies from gun to gun and with various bullet nose profiles mating with various feed ramps, but there are things we can do to make Set Back less prevalent. Straight walled semi-auto pistol cartridges are designed (the 357 Sig and 30 Luger have shoulders to head space on, so this does not apply to them, but only to straight walled casings) to head space on the case mouth, so you should not apply a heavy crimp to that case mouth, like you can with revolver cartridges. Instead, you can to two things; you can undersize the casing so that when a bullet is seated/loaded into that casing there is substantial case tension on the bullet shank, thus holding the bullet in place firmly…..not as firmly as a decent roll crimp though. Also, if you can find a powder with the correct density/burn rate, you can fill that casing with a powder that has to be slightly compressed by the seated bullet. This gives that bullet nowhere to “Set Back”. The bullet may still drive slightly deeper into the casing, but it will be slight and should not be sufficient to raise pressures. Of course finding a powder that will give the velocities you want at the pressures you can live with that is flash suppressed and fills the casing sufficiently, with a given bullet weight/length, can be very difficult.

In short do not keep rechambering the same cartridge over and over, unless you are willing to throw away that cartridge after a couple chambering’s.

Let discuss one very current and prevalent pistol/cartridge combination that causes dangerous Set Back. Years ago (as of this writing-11-01-15) Sig introduced the 357 Sig. cartridge chambered in their famed P229 pistol. The combination of the Sig pistol mated with the 357 Sig. cartridge works and works very well with almost zero bullet Set Back caused with multiple rechambering of the same cartridge as the magazine was designed to sit high inside the frame, so when the slide comes forward to pick-up a new cartridge, that cartridge is fed almost straight into the chamber with very little feed ramp contact. Sig designed this cartridge specifically for their pistol and did it well………Enter Glock! From the onset of this discussion I need to make it clear that I use and rely (stake my life) on Glock carry pistols, except those models of Glock that fire the 357 Sig. I am going to speak in very general terms on this subject, so I do not end up writing a book here. Glock basically used their standard designed 40 S&W pistol models to chamber (retrofit) in the 357 Sig. This design does not cause the magazine to sit high in the frame like the Sig. P229, so when that Glock slide slams home and picks up a cartridge from the magazine, it rams that cartridge into the feed ramp and whereas the Sig. P229 will not cause bullet Set Back, the Glock design exacerbates Set Back. This situation is due in part to the design of the 357 Sig. cartridge having a very short neck, which even if you undersize that neck prior to bullet seating, (and we do) it is not long enough to give a tension fit to the entire bullet shank and there is very little of anything holding that 357 Sig. bullet in place in the casing, thus it is easily driven back into the case upon chambering in the Glock models. Because of the way Glock pistols work (don’t work) with the 357 Sig. cartridge, we have considered not loading 357 Sig. ammo as the Glocks are cheaper than the Sig. P229 and therefore they have become very prevalent in the market place and I get tired of nasty emails accusing our 357 Sig. ammo of being prone to bullet Set Back in Glock pistols, especially by the folks that insist on rechambering that same cartridge over and over. Now, I need to be very clear again………Glock, like any maker of any product, will over time, modify (upgrade) their pistol models that fire the 357 Sig. cartridge. So what I have just written, will probably not apply to Glock pistols chambered in 357 Sig. forever. However, at this writing, (11-01-15) what I have written, does apply across the board to Glock pistols that are chambered for the 357 Sig. cartridge. Again, keep in mind that I love and rely on Glock pistols for many uses in many chambering’s, but not when chambered in 357 Sig., as of today. Do I need to say again, that I really like Glock pistols in all other chambering’s!?

Hopefully, this little essay has explained, in general terms, the dynamics of bullet Set Back and will be of some use to our customers.

As always, God bless and good shooting.

Tim