Friends don't let friends by Colt's. Bushmaster or just about any other AR clone is better than a Colt. The new S&W MP-15 is a very nice M4-type carbine. These are my beefs with Colt:
1. The stupid double headed screw for the upper and lower receivers. Everyone else uses the captive front and rear push pins. Much more sensible.
2. The oversize trigger and hammer pins (0.156 inch) versus the Mil-Std 0.125 inch diameter pins. The oversize pins mean only Colt hammers, triggers, and disconnectors can be used. No such problem with spares for other AR clones or their military counterparts.
3. The butchered back end of the Colt bolt carriers. Why Colt milled off the bottom side at the rear of the bolt carrier is unknown. However, it looks like hell and, I think, weakens the bolt carrier.
In the end, you pay your money and make your choice. I have three AR-type rifles all built from parts on Olympic/SGW lower receivers. They lock up tight with G.I. parts, and they shot just fine. I have owned Colts in the past, but I wouldn't have one of their current production guns. The quality isn't the best.