Here's a run-down on the gun, and the ammo:
https://gatdaily.com/review-sig-sauer-xm5/
From the article: "Yes, 5.56 works well but we have pretty much tapped it out. "
I've tapped out 5.56, for distance and accuracy. I've run 5.56 75gr handloads out to 1,050 yards, accurately, repeatedly. I've banged 1,100 yards with it, but it's losing serious steam there, and shooting conditions seriously affect when you'll make those hits, for 1,100. I've run it at 850 yards in a full-value 14.9mph wind, and it's 5.75 mils of wind hold to get it on target, and keep it on target - thank God for high-wind days, to learn that. At 850 yards in a no-wind condition, it's boringly repeatable. 5.56 gets it done, at distance, and projectile choice is an important factor in "what will happen" at distances like that. 70gr Barnes TSX projectiles rule, for that kind of distance, if you want something "hurt," or need to give something a leak.