The firing pin of the AR platform is a free floating pin. It's pretty normal, and I'd be more surprised about it not happening.
That said,
I chambered the same 4-5 rounds over the course of about 3 months, nearly every day in a turd-world-country... see what I did there... before I was advised not to, but the reasoning from the old crusty sergeant was that it could eventually misfire.
Doc Roberts, who is one of the foremost minds on the subject of ballistics and wounding had this to say in his research:
"A large SWAT team here had a failure to fire from an M4 with Hornady TAP ammo during an entry--fortunately no officers were hurt and the suspect immediately threw down his weapon when the carbine went click instead of bang. After the incident was concluded, the team went to the range and expended the rest of their carbine ammo and had one additional failure to fire. This same team had 3 Hornady TAP rounds fail to fire in training a couple of years ago. Last year, when Pat Rogers was teaching a class at a nearby agency, there were 5 failures to fire using Hornady TAP ammo. In all 10 cases, there appeared to be good primer strikes, but no rounds fired. On analysis, the ammunition had powder and checked out otherwise.
However, two problems were discovered. First, some of the primer strikes had insufficient firing pin indentations. The round from the potential OIS incident had a primer strike of only .013"the minimum firing pin indent for ignition is .017". In addition, the primers on the other rounds were discovered to have been damaged from repeated chambering. When the same cartridge is repeatedly chambered in the AR15, the floating firing pin lightly taps the primer; with repeated taps, the primer compound gets crushed, resulting in inadequate ignition characteristics--despite what appears to be a normal firing pin impression."The resulting answer is, you're not going to have problems with it misfiring, but you're going to have issues with it NOT firing after as the primer doesn't ignite as it should.
Source: DOCGKR (doc Roberts)
http://www.lightfighter.net/topic/5-56-mm-duty-loads
