With so many decent suppliers manufacturing AR15 parts I feel the deviation/difference in the quality between them has gotten so small that it's nearly indiscernible. That being said I feel there are a few things if you're building a carbine that don't cost an arm and a leg that for the small price difference can actually make a difference:
Hammer forged barrel (chrome lined)
Quality trigger group as it changes the feel of the gun (Rock River two stage is reasonably priced)
Bolt made from Carpenter's Steel #156
Any free float tube
Things I personally prefer:
Commercial buffer tube
NiB Plated Bolt and FA cut Carrier
EOtech sight - XPS1, 512, 511, whatever
Spikes Tactical H2 Carbine Buffer - very smooth and quiet
3 Point sling
Smooth free float tube
Troy forward vertical grip
My first M4'ish carbine was built by Fulton Armory (the M14/M1 experts). They suggested a mid contour barrel in 1/9, flip up YHM forward gas block/sites and a smooth YHM free float tube. I was hesitant at first about the 1/9 versus a 1/7.
Hk USP 45 Expert, USP 40, USP 9, SIG P226, S&W M&P9, Hk 4, Makarov, Desert Eagle 40, Beretta FS92 Centurion, Kimber TLEII 1911A1, Glock 22, SIG P225, 1943 Rem Rand 1911A1, S&W mod 64, Walther PPQ, SIG P229, Browning BDA 45 (SIG P220), Hk45, SIG P230
Last edited by sailskidrive; September 25, 2011 at 10:17 PM.