The only companies that have ever landed M4 contracts with the U.S. Government are the original Bushmaster which is owned by Remington now and has big time quality control problems, Colt and FNMI so if you want authentic M4 builders that's it. You'll pay a lot extra for the name with both Colt and FNMI. I'd be hesitant to say the quality is any better than Windham Weaponry who evolved from the old Bushmaster Company or many other builders now. My semi-auto M4 SBR is a mixture of old pre Remington ownership Bushmaster, Colt and LMT with all of the most recent SOCOM mods for reliability as assembled by myself. It's a flawless weapon with more ammo through it than I care to count. It's never missed a lick and is consistently accurate.