What to look for in an AR?
If you just want a decently built & reliable gun with reasonable accuracy, it's hard to beat a Colt 6920, Stag Model 1 or 2 (1 has a carry handle, 2 does not) or a Sig 400, IMO.
All will have a decent level of resale value.
If you want more, or something more specialized (like a 3G gun) or a stainless barrel, then the only limit is the depth of your pockets.
MM
