Ok, I've only had an AR for a little over 2 years and I've never used this ammo, what are the advantages of it over and above that it seems to be plentiful and is less expensive?
Plentiful and less expensive, along with it's corollary "available" should be enough. Being one of the first two U.S. Army ball designs for 5.56 (along with M193) made it very popular as a milsurp or replica round by a variety of producers. Third generation Mk262 is as of yet, not as popular.
1. AR-15s come in a variety of configurations and twist rates. For everybody that claims M855 is a "junk" round, there is another person that finds M855 more accurate than M193 in their rifle. I am one of those people.
2. Some people find value in enhanced M855 long range penetration. This was one of the main design requirements for the round.
3. M193 and M855 both have post-impact yaw as a wounding enhancer. Yaw can be inconsistant in both, based on velocity and range to the target, in addition to differences in individual rifles. That being said, both can be and are used as defensive rounds, even though it's generally accepted that expanding rounds are better in a civilian market.
Given their cheapness, using the same round for practice and defence is advantageous for some people.
4. M855 is not a match round. However, if your rifle shoots 62g ammo well, it is a good baseline round for practice and plinking (remember, available and cheap), while saving premium 62g ammo for special tasks.
