In a semiautomatic or fully automatic weapon the NATO chamber is more reliable than a commercial .223 chamber. If your rifle is going to be run hot, dirty, and in adverse conditions the NATO spec chamber is your friend. Don’t just take my word for it, consider that the largest operators of 5.56 NATO rifles/carbines seem to be pretty set on it as an actual specification. Probably because having rifles and carbines (not to mention light MG’s) that feed, fire, extract, eject, and feed again as reliably as possible are pretty handy. Much more so than a temperamental range play toy with a tight chamber. The gun needs to work first and foremost.
I personally see no issue with free bore. Currently I own 4 carbines or rifles with 5.56 NATO chambers, and all of them shoot quite well even with the free bore inherent in the NATO chamber. Light weight 55gr bullets pose no problem even in the faster twist these barrels run. My 3 most shot are a Colt, and 2 Daniel Defense barrels, all 3 have turned in enough sub MOA groups with 55gr up to 77gr bullets that I don’t think it’s a fluke at this point. The 4th example, a Knight’s Armament SR-15, hasn’t been evaluated as carefully yet since I just got it, but shows some promise even with inexpensive 75gr Hornady Steel Match and a 1-4X24 for optics. Heck, it’s a hammer forged chrome lined lightweight barrel too and the dang thing appears to be a shooter.
I understand the reluctance of some to embrace my reasoning, and 15-20 years ago I’d have agreed. A decade or two ago a production rack grade 5.56mm NATO chambered barrel with a 1:7 twist would probably not perform as well as a production rack grade .223 Remington chambered barrel with a slower twist rate. Particularly if the NATO barrel were chrome lined. However, times change and so does technology. Bullet technology is better, so these days modern bullets of lighter weight do just fine in faster twist barrels, likewise many of these newer bullets don’t seem to be negatively affected by freebore very much. Barrels are also a lot better, 17 years of conflict and lots of .mil orders have pumped a bunch of $$$ into small arms technology. Current mil spec chrome lined barrels are so much better now than 15-20 years ago in the accuracy department that it boggles the mind. Be they hammer forged, or button broached the barrel making world has figured out how to produce barrels out of tough CMV 4150 with NATO chambers, chrome line them and still have them shoot very very well. Maybe it’s tighter QC, better stress relief, more refined chrome lining processes, or all of the above. Either way the results are pretty awesome for all of us.
So that is why I think the industry should just cut NATO chambers for most of these rifles regardless of action type. You give up very very little practical accuracy but gain in other areas significantly.