The 11.5" barrelled AR was a Colt that had been built up by a Phoenix SWAT team member. An armorer taking the class suggested that the gas port under the front sight probably needed to be enlarged to allow more gas to flow into the gas tube. He said that probably would fix the short cycling problem.
A couple things I learned about equipment in the class:
1. Keep the toys on the rifle to what works and no more. It was amazing how heavy an AR could get after having it slung for 4 days. Giles said I had a good set up... Colt AR15 A3 Tactical Carbine 16" with Giles sling, Aimpoint and M3 bayonet mount (built by Ken Elmore at SAW in Arizona...same guy who built Giles' rifle). He suggested I consider replacing the HBAR with one of the lightweight barrels to shave a pound or two off the front of the rifle.
2. Carry a spare parts kit (like the one Bushmaster sells) with all the stuff that's likely to break. Giles said, better yet, to carry a complete bolt and a parts kit. One rifle broke a firing pin (only found part of it) and others had transient failures that may have been related to inadequate lube.
3. Magazines were my biggest problem. Most of mine are surplus LEO Colt mags. Apparently, I had at some point dropped one and it slightly bent in one of the rear tabs. The top round was held slightly ahead of the others and it double fed on me (on the firing line while I was being timed). At least I got my chance to do a clearance drill under pressure! I also had two LaBelle mags that were just out of the plastic bag and were marked as being manufactured in 1988. These appear to be teflon coated? Anyhow, they worked for about two or three loads and then the follower would bind when loading them. Maybe they got dirt in them or something. I'll look at them this weekend and try to figure out what happened. Another Colt mag (dated 03-04) loaded fine and doesn't appear to have any other problems, but would jam my rifle. I'll take a look at this one too and let you know what I learn. I took 10 mags to the class and had 4 of them choke on me. Before I take my next carbine class I'll going to take every mag I plan on using to the range and fire at least one loading out of it. Once I set these aside my rifle ran fine. It was sure frustrating to do a tact reload and get a double feed! I'll probably remove the followers and springs from these 4 mags and destroy the bodies if I can't fix them.
4. I also learned that the Aimpoint was superior as a sighting system to the other red dots used in the class. Having 10K hours on a battery meant I could turn it on and leave it that way for 4 days (other red dot folks were switching theirs on and off and sometimes forgetting to turn them back on). I heard the new Aimpoint ML3 has something like 40K hours on a battery...nice! The Aimpoints used in the class were 100% reliable in the dust and heat and students with them did especially well in the night exercises.
5. Get a good weapon-mounted light that allows you to identify an adversary at 35-50 yards. I'm using the M3 (which cost about $110) and there are light systems that work for ARs costing from about $100 to more than $800. This is a necessary item if you can ever imagine needing your rifle in a structure in the day or night. The light will help you identity the target and possibly blind the target.