All AR15 rifles have a floating firing pin. The firing pin strikes the primer every time a bullet is chambered. And this strike will leave a small indention on the primer, slightly damaging it. If you chamber the same round 10 or 20 times, you have significantly damaged the primer (and brass and bullet) and its very likely that the round wont fire when you need it to.

In a rather well known incident in the tactical community, a SWAT cop continuously chambered the top round in his magazine during raids. After the raid, he would eject the round and put it back in the top of his magazine. Well that round had been chambered 10 or 40 times (who knows) ....and he got into a shootout. His rifle went "click" instead of "bang". Because the primer was damaged due to his repeatedly chambering the same round.

Never chamber a defensive round more than twice. Once that has happened, rotate that round to the bottom of your magazine, or add it to your training ammo to be shot up at the range.