Q. What about XM193 from Federal and Q3131A? I have heard that some of these don't meet military specifications, particularly with respect to sealant on the necks and primers?
Most manufacturers who market "Mil-Spec" M193 like ammo are either taking "factory seconds" that would otherwise go to the military and packaging them for civilian sale or reducing the second inspection of rounds before distribution. Ammo destined for the government is tested in lot batches and the entire lot is rejected if the batches fail spec tests. Generally, this ammo is still excellent for both plinking and defensive use. XM193 and Q3131a in particular are exceptional rounds for all around civilian use and still show very reliable function in AR15s.
Some AR15ers have noticed, however, that sealant or other small details are sometimes lacking for some lots of these rounds. Sealant in particular is not a critical component to average civilian sales and therefore if sealant problems develop in M193 rounds destined for military contracts (and therefore out-of-spec rounds) they are usually sold as civilian versions of M193 (e.g. XM193). It should be pointed out that this is no reason whatsoever to avoid these rounds.
If you are really concerned about sealant or intend to use the rounds for long-term storage or quasi-military use where they are likely to see harsh and moist conditions do some testing on random samples of your lot for sealant. See also the testing done below in the Ammo Oracle.
NATO stamp on a Lake City 5.56 round from