I agree about the Colts working fine before the feed ramps were installed on the M4 upper receivers. I run a number of M16's on full auto and have never had a problem. I got the M4 upper with feedramps but did not notice any difference, but then I was not using .223 hollow points, which is what the feed ramps were supposedly put there for. Then one day, with one of my 11.5 inch Colt barrels on a Colt Commando upper receiver, I had the rounds hit the upper receiver just below the ramps in the barrel extension and stop dead. I could not believe it. This was with Winchester FMJ ammo, which is what I always used. I tried different mags and it kept happening. I then used a Dremel Tool to put feed ramps in the Commando upper receiver to match the feed ramps in the M4 upper and have not had a problem since then. So, I am not sure why I suddenly had that problem, but the feed ramps did take care of it. It made a believer out of me, but I have not heard of others having the problem with feeding like that. But I now have the feed ramps in all of my upper receivers. Better to be safe than sorry.
Alamo Professional Arms