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 1/9 twist. The chamber and bore are chrome lined. The barrel is made of 41V50 steel. We have also used a A2 flash hider on it. C7 Upper receiver is black finished. Handguards are surplus USGI Mil-Spec.
Take a look at this great article about our upper http://www.gunsandgunsmiths.com/retro-ar-project/
This item carries a lead time. Typically it is 2-3 weeks. Due to product availability lead times may vary.