Here is one of our favorite custom builds. We decided to try and rebuild the old school C7 M-16A1 Carbine These are using a true M-4 carbine barrel with a 1/7 twist. The barrel is made of 41V50 steel and the chamber/bore is chrome lined. C7 Upper receiver is black finished. Handguards are single wall Mil-Spec.
The 80's and 90's saw major changes to the modern duty rifles carried by US forces. Most notable the adoption of the carbine gas system and shorter barrel over the longer rifles. These advancements where directly related to the requests from special forces units such as SOF-D, Army Rangers, and Navy SEALs. The 723 and 727 were the precursor to the modern M4 we all know today.
Upper: TNTE 7075-T6 C7 with Black Type III hard coat with M4 ramps
Barrel: TNTE 14.5" 5.56 1/7 Chrome Lined barrel with 1/2x28 thread
Gas System: Carbine
Gas Block: Pinned on standard height A2 FSB
Handguard: Two Piece Carbine
Flash Hider: A2
Charging Handle: Optional
Take a look at this great article about our upper http://www.gunsandgunsmiths.com/retro-ar-project/