XM193 is a military designation for 55gr. 5.56mm ammo. It comes in 840 round lots because that is how many rounds fit on stripper clips in an ammo can.
I can't attest to the reliability or accuracy. It is a lower tier ammo, although I have used it some and I haven't had much problems with what I've uses. Most M193 and M855 I've fired were military with NATO or WCC headstamps.
The American Eagle is kind of junk. Sure you get some more, but so what. I guess if you just want some close range plinking, go ahead. I don't hear much good about AE, other than it is a little better than the Russian garbage.
What I do is load and reload. I get 1000 rounds of virgin primed NATO military brass for $137 and I get new SS109 projectiles (not pulled) for $85 per 1000. So all I have to do is add powder and I get new M855 loads, but more accurate. I can also load Mk262 also. All for much less than what it sells for, and I get much better ammo.
Because I load, I can afford to shoot more, and I can shoot stuff like the Grendel, Beowulf, Blackout, etc., and not break the bank.
But if you are looking for good loaded ammo already, try and find the stuff with NATO headstamps. Look at gunshows maybe. The XM193 is good plinking stuff, the M855 is better in my opinion, particularly if you have a slightly faster to fast twist rate.