It seems here was the best place to post this. While cleaning my M&P 9 I noticed the top round in the mag had the bullet set back a bit. Upon closer inspection, I found the round that had been in the chamber also was short. The rest were fine. The ammo in question is Hornady Critical Defense 115 gr with a XTP bullet.
What would cause this. These rounds are not in the gun when I shoot, I use my reloads for target practice. I carry with a round in the chamber, and when I re arm the gun, I let the slide forward to chaP
Re: OP. Set back is the result of the violence of the loading cycle that is common to how semi-automatic pistols function. As the cartridge is fed from the magazine to the chamber then nose of the bullet impacts the feed ramp and sometimes elsewhere in the loading cycle. Taking the same round and repeatedly loading it in the magazine and then chambering it will sooner or later loosen and set back the projectile of any cartridge every made. I've seen in not just in 9mm but also in .40 S&W and .45 ACP.
It is understandable that one would not want to fire expensive and often hard to obtain high performance ammunition that has been bought primarily for SD/HD and not range use. However repeated cycling of HP ammunition will lead to set back just as with common range/practice ammunition.
One way to avoid the problem is to not cycle the same round through the pistol. When you clear the pistol take the round that was in the chamber and set it aside.
Empty the magazine with the rounds laid out as they were in the magazine. Load the round that was in the chamber as the first round in the magazine. Now load the magazine with the rest of the rounds beginning with the round that was previously last in the magazine. The result will be that each time you clear/clean, etc. the pistol, a fresh never cycled round will be chambered, all cycled rounds will be in the bottom of the magazine having been cycled only once, all uncycled rounds will be at the top of the magazine.
It is exceedingly unlikely that a single cycle through a pistol would cause any significant set back. Perhaps after two cycles, a round might just possibly begin to show set back such that one would be concerned. Most likely it would require more than two cycles. With a typical high capacity magazine, cycling all the rounds would mean one had cleared/cleaned the pistol between 30-34 times. That would greatly extend the useful service of expensive SD/HD ammunition such that one could than set those possibly compromised rounds aside for range/training use. In the time it took to cycle box of SD/HD ammunition through a magazine twice, one would likely be able to locate and purchase replacement ammunition of one's choosing.
Just thinking out loud. HTH. Sincerely. brucev.
Thanks
