I was at the range today and fired about 3 1/2 magazines worth through my rifle. When I cleared the rifle, the live round that I removed from the chamber had a small dimple on the primer, apparently from the firing pin. 
This round was chambered after firing the previous round, but then unloaded without a pull of the trigger. Is it normal for the firing pin to ābumpā the primer upon closing but not fire the round?
Perfectly normal.
The firing pin on an AR is not spring loaded and when the bolt goes forward it will often leave a mark on an unfired cartridge in the chamber.
Current recommendation is to dispose of the round (firing it? 
) after no more than 3? chamberings.
I donāt often have a requirement to clear my weapon so I donāt usually worry about it but ā¦ if I do wind up clearing then the top round gets mixed into the unloaded rounds to reduce the chance of it being chambered twice.
Yes, perfectly normal. The firing pin travels forward under inertia but not hard enough to set the round off.
Iāve been told not to use them for duty after chambering and to shoot them in training.
Thanks for the replies. Itās just range ammo, so Iāll throw it back into the mix.
I believe that in an edition of PS magazine the Army was advising that if a round has been rechambered 5 times, it should be discarded. I am sure at some point I did it more than I should have.
Absolutely normal.
normal it happens on my rifle and every one elseās nothing to worry about
I would file this under the āgood practiceā folder.
[deadhorse]
As would I.
I ran a test, and got 7 re-chamberings out of a round and it still managed to fire.
UNSCIENTIFIC
SAMPLE SIZE=1
ROUND COUNT=1
Having said that, My advice, and practice, is to cycle the ammo out to the training pile, at least for ammo and guns used for social purposes. My duty ammo costs $0.80/ea, considerably less than my life.
[/deadhorse]
One of the issues with rechambering is having a round sit in a hot chamber.
Helpful info/photos in the 5.56mm Duty Load sticky