I recommend buying a brand that is known to build their rifles to mil-spec. That way you know they at least built it to a certain quality. I went with Spike's Tactical 16" CHF enhanced upper w/NiBX BCG. I havent regretted it. Other makers that are known to build mil-spec are: Colt, Daniel Defense, BCM, LMT, and a few others I cant think of right now.
Glock 20SF Gen 3
Glock 27 NiB-X Gen 3
Glock 19 Gen 4
Spike's Tactical Enhanced CHF 16" Mid-Length M4
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