A friend of mine was having an issue with his Glock 43x running brand-new Shield Arms 10 round magazines. I’d run into this on new Shield Arms 15 round mags for the 48x in the Alpha Foxtrot S-15 from, time to time; as well as the Kimber CDS-9 for new Kimber 10 round magazines.

After playing around with my grip on the S-15, I’d figured out there was just a hair of play on the magazines and if I was squeezing too tightly with the grip hand, it was causing the magazines to sit just a hair downward at the front. It looks like the last round (and it’s only the last round), acts on the slide lock like it’s empty. Relaxing the grip a bit (which is good for preventing shooting toward your support side) got rid of the problem. After the mags broke in, the problem disappeared entirely.

Tried it with his Glock — shoot teacup style with my pinkie out just to make sure — functioned fine through several magazines, but if I was gripping tightly, it locked back on the last round. This occurred even with underloaded magazines — so seven or eight in the mag still caused the malfunction. Now hitting the slide stop did send the last round into battery, just fine.

I’ve seen this on my Kimber CDS-9, but only on the replacement extra 10 rounders I bought to be legal traveling through the more authoritarian states. Same thing — don’t grip so hard, it didn’t occur. When it did, hitting the slide stop sent the last round home. I don’t use the extra 10-rounders enough to have gotten the springs a bit looser, but I suspect the problem will work out after a few dozen trips through the gun.

If you’re having the same problem, try this fix. Might work, might not. You can always buy more magazines and hope for the best.