I've had trouble with some fast burning powders in mine. Powders like Benchmark, H322, and Reloader-10x. The problem isn't that those powders aren't accurate in my AR, but that because they burn so fast, they're all spent by the time the bullet reaches the gas port in the barrel. This causes the bolt to not cycle back far enough to grab the next round off the top of the magazine. The worst combo I tried was Reloader-10x with the 32gr V-max bullet. That fast burning powder, and the very light bullet, may have combined to send comparatively little pressure into the gas tube. Another possible issue could be that the gas port in my barrel is too small to allow enough pressure to be sent into the gas tube, and the bolt doesn't cycle fully. The manufacturer suggested they could open up the gas port on this barrel, but they said that could cause problems in itself, and there's no going back.
One other possibility in the cycling problem described above, is that when I tested with the powders mentioned, my AR had a very low round count. At this time, I was also not using as much lube on the bolt carrier group as I do now. In other words, the action may have been broken in, or been kind of "stiff". I now use much more lube on the BCG, and have noticed much smoother operation. I've also replaced the factory buffer spring with a Tubbs flatwound Chrome Silicon spring. This is supposed to increase lock time by a small amount, and last 100 times longer than the factory springs before their performance degrades. I doubt I'll get 5000 rounds down this tube, but the spring is in the lower receiver, not the upper, so maybe I'll hit that on the next barrel. One other thing I didn't get to play with much last year was the Tubbs Carrier Weight System (CWS). This is a cylinder and 2 weights that get slid into the Bolt Carrier Group. You can use the cylinder by itself to add just a bit more weight to the BCG, or add the weights too, which are made of different weight metals to give you a variety of total weights. I won't get into everything about what they do here, but I picked some up, thinking they might help my cycling problem. (If you are thinking what I was, that adding weight to the BCG should not help it move any easier with low pressure, we are on the same page. The thing is, the description of the product says that it actually works opposite of that. What happens is it keeps the bolt closed a little longer, developing more pressure to be sent down the barrel. In turn, you get more pressure coming back to cycle the action. It seems like they would cancel each other out, but they weren't so expensive I couldn't try them. I have no conclusive results yet.)
I'm still a bit frustrated when I hear reports all over the Internet of guys getting awesome velocity out of their AR-15's using the powder/bullet combos that fail for me. I'll probably go back and try some loads with those powders again this summer, but have actually found a powder that works beautifully with my AR in Ramshot TAC. Hodgdon Varget is another great powder I've tested with. H4895 works great too.
I guess in general, powders that burn clean may be preferable in an AR-15. A dirty powder will cause you to have to clean more often. I mean cleaning the Bolt Carrier Group. For normal quick cleanings, I just clean the chamber and bore, which is no more than cleaning a bolt action. With the BCG on the other hand, you can when when powder residue and gunk start to combine with the oil, and make it "sticky". That's one reason I have somewhat settled on Ramshot TAC. It burns really clean.
Those are the main issues I've come across with my only AR so far. Don't let them alarm you. It seems most guys just load up whatever they want and it just works in the AR-15. Mine seems to be finicky.
My next mod for this rifle may be an adjustable gas block. Open up the port, then use a gas block that can regulate how much gas gets through to cycle the action, depending on different bullet weights or powders. This is taken from an article on the new Ruger SR-556 (which is a different system than the AR, but some same principles apply),
"The four-position adjustable gas regulator allows the shooter to tune the rifle to specific ammunition" I might try something like
this.
