Bullet setback

[kalash]

I unload my HD Glock 19 fairly often - range trips, dry firing, etc. This got me wondering about bullet setback and just how unwise it is to keep chambering the same two rounds over and over (I say two because there are two that rotate between being the chambered round and the top-off-the-magazine round). The best way to do this? Well, measure it! This data is for Hornady Critical Defense 9mm rounds. The first two rounds, as I mentioned, have been chambered probably a dozen times each by now. Strangely enough, the two rounds that have repeatedly been chambered came out longer than all but one round in the mag. Repeatedly chambering this particular ammo in this particular gun doesn't seem to result in any bullet setback whatsoever. What do you make of this?

Geekiness ahead...

Average measurement was 27.41mm with a standard deviation of 0.078mm

Edit: the difference between the shortest and the longest is only 0.27mm so it really doesn't seem like I need to worry about setback at all. Also, every chambering is done by either releasing the slide lock or using the slingshot method; I never guide the slide forward.

 

[MadRussian]

I'm going to have to measure mine to see. Same make of gun and ammo, but in .40

 

[YZhou4]

It really depends on the particular combination of ammo and firearm. Just by looking at Critical Duty ammo, it appears less susceptible to set-back due to the bullet geometry (relatively sharp angle with the polymer tip, reduces friction and impact on feed ramps probably) and the aggressive crimping/texture where the bullet meets the casing. I have also measured several brands of 9mm HP ammo that I have re-chambered repeatedly (Critical Duty, HST, Corbon DPX, Speer Golddot) and have not found any measurable set-back. A larger caliber, lets say .45, with a bullet that has a large open hollow-point cavity, in a gun that has a very angular feed ramp, would be more likely to induce bullet setback probably.

That being said, every reputable trainer I have ever taken a class with has recommended not re-chambering ammo that you might be using for self-defense. There are well documented cases of ammo failing due to set-back and/or other effects of repeated re-chambering (such as cracked/broken primers), almost all of them from law enforcement involved incidents. A good practice is taking the once or twice chambered round and putting it into a box to shoot on the next trip to the range, that way you get some practice with your carry round each time you shoot your carry gun.

I have fallen into a lake with several magazines of HST, dried them off half and hour later, and they shot fine 2 weeks later. Does this mean I will decide not to replace all carry ammo that gets really wet if I can? NOPE! In all likely-hood quality self-defense ammo will work even after being re-chambered multiple times and dunked in water, but why take the risk? Good rule of thumb is to shoot off all your carry ammo every 6 months.

 

[Al-Jim19]

I chamber each round 4x, and mark the base with a sharpie each time. After each round in the box has 4 dots I go to the range and practice with them. Avoids setback, lets me get a little practice with carry ammo, and keeps my carry ammo pretty fresh which I like.