What kind of ammo is everyone using in these? Especially the guys saying that their's shoots MOA? I'm using Federal XM193 55gr and am getting 2.5-3 MOA with the odd 2 MOA group. I don't reload so what ammo should I try if I want to get some tighter groups?
That XM193 is shooting about where it should for the parameters of its construction.
Federal Gold Medal Match is right around the gold standard for a few calibers, including .223/5.56. If it can't shoot GMM at least quite well, it might be a dog (but not necessarily a guarantee).
After paying for GMM, you will quickly realize that if you want to chase accuracy, hand loading is the way forward, unless you have deep pockets.
If it doesn't shoot GMM well, I'd be tempted to blame something other than the ammo (cleaning, crown, how it is mounted/fastened in the stock, rings, scope mount, and lastly shooting position) and try, try again.
Also, while it is great to see some folks exclaim stuff like 'Hey, I shot a half minute group today!', take some of the claims of sub MoA (at least ones where the owner is claiming it is consistent) with a grain of salt, especially if it is past 100m...