Not sure about the Colt disco but if it works properly I wouldn't mess with it. You can test the trigger function with no upper on but be sure to catch the hammer with your thumb, don't let it slam against the bolt release. Cock hammer, pull trigger (keep it pulled back), catch hammer with thumb, recock and see if the disco catches it, then release the trigger and see if the trigger catches the hammer back while the disco lets go. Do this repeatedly until you're sure the trigger function is ok. You can test this against your RRA for proper function.
The biggest issue with assembling an AR isn't so much getting the parts together right - it's getting the right parts together.
US Army 1966-69, VFW Life Member, Retired Geek