rhatland, fire control parts refers to the trigger, hammer, disconnector, pins, safety and springs. Colt typically used the non Mil-Spec (large - .169) pins for their commercial rifles. Bushmaster on the other hand has always used the Mil-Spec (small - .154) pins in their lowers. Therefore, if you have large pins, which I am almost certain of, you cannot use Mil-Spec pins in your rifle. Colt also used the screw type front pivot pin in both the large and small versions. To make matters worse, Colt did not drill most of their lowers for the detent and spring, which holds the standard front push-pin and prevents it from being pushed out of the lower. You will need to check your lower to see if it has been drilled. If so, you will find a small hole drilled into the mag well between the front pivot holes on the lower, near the right hand pivot hole. If there is no detent you cannot use a Mil-Spec pivot pin. Your options are to have the lower drilled, stick with the Colt screw model or use one of the after market pins with a built-in detent.