[Grizzman]

I've read some negative comments on M4Carbine.net about Sharps Rifle Company BCGs, with one individual stating that he's read of a bunch more. Considering the fully vetted nature of Colt, LMT, and BCM bolt carrier groups, I see no reason to stray from these for a serious build. They're readily available for around $180. They haven't been around as long as the above, but Sionics and Centurion both offer great BCGs, with Sionics units costing less than the others.

Geissele trigger groups are fully awesome, and I doubt you'll find many people that would disagree. Don't let the low price of the LaRue MBT-2S fool you into thinking it's not worth considering. I've got quite a few of them, and they compare very favorably against my SSA.

The easy buffer/spring answer is to get an H2 and Colt or Sprinco Blue spring. The Sprinco should greatly increase service life, but Colt springs are cheap enough to replace multiples before reaching the $22 price of the Sprinco. I may have read that Geissele's spring is only supposed to be used with their buffer.

A lot of people swear by the A5 system, and it does work smoothly. I've got more A5s than carbine receiver extension, and most of the carbines are part of 1st generation Magpul UBRs.

Today, BCM has just released their enhanced upper receiver. It's got thicker than standard walls for strength, differences in internal shape to modify gas flow (for suppressor usage), etc.

Since the release date wasn't known, or if they'd even be offered as a stripped receiver, I ordered a VLTOR MUR-1A late last week for a precision-ish 6.5 Grendel build, and it arrived today. I'm not going to finalize the upper assembly until a friend arrives for the holidays, but the machining and finish are definitely top tier.