Longest I've ever pushed a 5.56 was a little over a mile, using Mk262 MOD 1 out of a Mk12 MOD 1. Long and short of it was I had a bit of time one afternoon in Afghanistan and wanted to push it. This particular Mk12 was legendary as accuracy goes, a legit 1/4 MOA, by far the best shooting gas-gun I've ever had my hands on. From 0 to 600m it was a one shot, stop rifle as bad guys are concerned, even in some pretty hellish conditions of 18-20 mph winds, I was able to connect with solid center mass hits. 600m to 800m I could pretty much guarantee hits in good conditions, only question was how long it'd take for them to bleed out. At 800m to 1200ish it ranged from about 50% hit rates to being able to professionally scare the shit out of bad guys.
Anyhow I'd say at our altitude (7,000-9,000ft) while pushing the 77 SMKs at 2900ish, it could easily hold 2 MOA at 1000m with some regularity under ideal conditions once the wind was figured out. Of course the wind is main problem with 5.56 at those distances, a butterfly farts and you are off one edge or the other, but if you can catch your splash and your fast on the gun then hits on man sized targets are quite possible.
As for lobbing 5.56 at a mile, it was mainly for the fun of it. My partner and I started on a bolder that was maybe 3-4 MOA at 1540m, wind was maybe 0-2 mph down low and after a little right and left we trued up on the 1540m rock. Wanting to see how far we could go and with time and ammo to burn, we lased a rock face halfway up a mountain at about 1700m. The first 4-5 shots were unobserved, then finally my spotter caught a little puff off the bottom and tried corrected me on. The problem was the vertical spread was at least 4 MOA, never chron'd the gun but I'm guessing SD's somewheres between 10-20 FPS. And of course the wind, even in great conditions, even being fast on the gun just wasn't fast enough with those long TOF all the way across the valley. We'd land 2 or 3 rounds on the rock face in a 4 MOA group, then send one unobserved off the rock face. We fired a total of maybe about 30 rounds and called it good.
In all we had a good time lobbing rounds, but it was certainly interesting and gave us an appreciation for what that system could and couldn't do. For kicks, I'd like to try it again someday but with better round and a more suitable location. I'm not sure the results would be astronomically better but I think cleaning up the SD's and having a nice big back drop would make it easier.
