6.03mm Tightbore inner barrel
Pre-upgraded version 2 gearbox
High density polymer construction
If you are looking for a base gun for your dream custom AR or you like the classic look, the Delta Armory DA-C01 CHARLIE™ M4A1 is what you need. This M4/AR-15 AEG is styled after the original M4A1 rifle of the 1990s but with some 21st-century ergonomic enhancements to improve handling and a styled-up Delta Armory receiver matching modern real steel "billet" receivers. 
Being a replica of the Delta Armory CHARLIE™ line, the DA-C01 is constructed primarily from high density polymer with alloy trim parts and is kitted out on the inside with a tightbore inner barrel, a rotary HOP-up unit, and a pre-upgraded gearbox featuring an SHS tappet plate and piston, 7mm bushings and a quick change spring guide. 
With more pre-upgraded internals than a platform this price deserves and some stylish, modern enhancements, the DA-C01 would make a fine basis for a custom build but can also be left as it is to take advantage of some interesting old-school features!