Q. Will Military Ammo wear my favorite National Match/Elite Sniper/$5500 accurized AR rifle out faster?
All shooting will cause wear. Each round you fire wears the barrel a little more and therefore will have some impact (however slight) on accuracy. This said, most barrels have a lifetime of around 10,000-15,000 rounds without much impact on groups. Your mileage, obviously, will vary. You can expect it to be a bit higher with chromed weapons, lower without.
Match rifles often have tighter twists (and therefore more friction or wear) than non-match rifles. Match rifles generally don't have chromed barrels or chambers. Your chamber, if its a match chamber, is likely to be the larger issue. Tolerances in match chambers are tighter and more sensitive to wear. Old M16's use to show throat wear after as few as 2,500 rounds.
All of this contributes to make wear a larger issue in match rifles than in non-match rifles.
Obviously, higher velocity rounds will cause more heat and more wear on your weapon. 40 and 45 grain "varmint" rounds are particularly brutal to barrels because of their extremely high velocities (up to 3600 and 3700 fps in some cases). By the same token, Mil-Spec rounds probably cause slightly more wear than lower velocity, lower pressure commercial rounds. Certainly, if you are worried about wear you will want to avoid steel/nickel jacketed ammo, particularly in non-chromed barrels. Such bullets were intended to be used in chrome-lined military rifles and may cause accelerated wear in non-lined barrels.
Remember that in addition to wear from use, cleaning, and particularly over-cleaning, causes quite a bit of wear. The wear in some obsessively cleaned rifles will exceed any wear from firing.
Also, sometimes what appears to be barrel wear-related inaccuracy in chrome lined barrels can be cleared up by cleaning out the copper build-up "fouling" in the bore with a good copper solvent.
Opinion: It all comes down to personal experience and preference. The highest tiers of competition replace their barrels multiple times in a shooting season. "Serious" competitors might expect a barrel replacement each year. How serious are you? How often can you afford to change out barrels? If you're THAT serious you'd stick with the same match ammo in that weapon that you use to compete and keep the round count very low. If not, then perhaps a few thousand rounds of surplus ammo in a year won't matter much.
Opinion: Bushmaster indicates: Mil spec. SS109 ammo will not measurably increase barrel wear under semi-auto fire and our mil. spec. (chrome lined) barrel will outlast any sporting rifle barrel - period. More barrels are ruined from over cleaning - or careless cleaning - than are ever "shot out". Chrome lined barrels really only need to be detail cleaned when the groups start to suffer. Otherwise, a little powder solvent (or "Break Free" with CLP), and a few passes with a brush, clean the chamber well, dry everything off and apply a very light coat of "Break Free" or "Rem-Oil" and put it away. We have had barrels here go 20,000 rounds and still be within mil. spec. when treated this way.
One FAQ author has a chrome lined Bushmaster that has seen 30,000+ rounds without a change on the upper that still shoots ~1.5 MOA at 100 meters.