5.56 or 7.62

[benEzra]

If you have the skill to use it to it's ballistic effectiveness, you could wreak havoc on a group armed with poodle shooters or kalashnikovs by staying outside their max effective range and delivering well-placed shots at the "Rifleman's cadence" of one shot per breath.

Effective range of 5.56x45mm Mk262 77gr OTM is at least 700 meters (765 yards). I doubt a typical M1A would outrange that under realistic conditions; that puts you in the realm of specialized long-distance optics and tuned match loads.

http://en.wikipedia.org/wiki/5.56x45mm_NATO#Mk_262

Mk 262

The Mk 262 is a match quality round manufactured by Black Hills Ammunition made originally for the Special Purpose Rifle (SPR). It uses a 77-grain (5.0 g) bullet that is more effective at longer ranges than the standard issue M855 round.

Two versions of the round have been procured to date. Initial production runs, designated Mark 262 Mod 0, lacked a cannelure. Subsequent production, designated Mk 262 Mod 1, added a cannelure to the bullet for effective crimping.

According to US DoD sources, the Mk 262 round is capable of making kills at 700 meters. Ballistics tests found that the round caused "consistent initial yaw in soft tissue" at 300+ meters. Apparently it is superior to the standard M855 round when fired from an M4 or M16 rifle. It evidently possesses superior stopping power, and can allow for engagements to be extended to up to 700 meters. It appears that this round can drastically improve the performance of any AR15 platform weapon chambered to .223/5.56mm. Superior accuracy, wounding capacity, stopping power and range power has made this the preferred round of many Special Forces operators, and highly desirable as a replacement for the older, Belgian designed, 5.56x45mm M855 NATO round.[34]

 

[HorseSoldier]

Effective range of 5.56x45mm Mk262 77gr OTM is at least 700 meters (765 yards). I doubt a typical M1A would outrange that under realistic conditions; that puts you in the realm of specialized long-distance optics and tuned match loads.

I can't speak for what the round was doing ballistically out at that range, but I've seen a first round hit on a steel chest plate at about 1200 meters through an M4 with Mk 262 during a class. (This included a spotter lasing the precise range and then figuring wind and drop with a ballistic program on a PDA, but that's still either real lucky or sub-MOA ammo.) I've also seen the spotter on that bit of messing around (who was one of our SOTIC instructors) make consistent hits at 800 meters with iron sights when we first got issued SPRs and Mk 262 to go with them. As far as I could tell, shooting a bunch of it myself, it will outperform the accuracy capability of most issue M4s and most shooters very reliably.

 

[C-grunt]

I was issued M262 along with my M16 SDMR in 2005. We routinely shot it out to 400 plus meters with not much effort. At 600 meters things became a bit tricky and at 800 meters it was downright hard.

I might not have been making first round hits at 800 meters but I would definitely ruin your plans.