".........that attaches to a standard A2 flash-hider."
Keep in mind that this means a true milspec flash suppressor. Some of the aftermarket look alikes (Bushmaster for one) arn't made to milspec dimensions and will not work with these suppressors.
Another kind of interesting twist: the AAC M4-2000 uses a special flash suppressor that mates with the suppressor. This means that you have to have either a pre or post ban gun (not during the ban since you can't remove the "muzzle brake" or you have to have it threaded). These flash suppressors are currently expensive: about a hundred buck each. If you have a number of ARs like I do, then you are talking about some real money in flash suppressors. BUT, AAC is supposed to be farming the manufacture of these flash suppressors out to a third party contractor who is supposed to be able to make them a lot cheaper than AAC currently is. In addition, you are going to have a choice of three different styles of flash suppressor including a Phantom. In other words, you are going to be able to have your cake and eat it too. You can have a good, effective flash suppressor, AND be able to mount a suppressor to it.
A few random thoughts: ask about the weight of the suppressor. As was mentioned, they are heavy. Mine (M4-2000) is rated for unlimited full auto. Since I am not shooting it full auto at all, I wonder if I could have gotten a lighter suppressor that isn't rated for full auto ?
My suppressor shift zero about 1 minute at 100 yards. It is repeatable. IMO, on an AR that might be used for personal defensive type applications, that shift is not worth talking about. From field postions, that is closer than you can hold (probably). If you are firing it from a very accurate platform with a big scope, from a bench then you can simply put a minute of elevation on your scope.
I took a formal carbine class this past weekend. Near the end of the class we went out to a range that has steel silhouette targets going from about 200 yards to WAY out there: over 1k. I was shooting an AR, with an Aimpoint, Wolf ammo (55 grain) and the suppressor. I was shooting from monopod prone. I got consistent hits out past 300 yards. At 400 and beyond, the ammo was not consistant enough (hits all over the place with a consistent sight picture) This was without any sight correction for the suppressor. Of course we would like to be putting all those shots in the chest cavity, but at 300+yards and an Aimpoint (4 minute dot) exact shot placement becomes an issue: hits on steel make me happy at those ranges. FWIW, on paper (silhouette) I put 5 out of 5 right in the "A" zone during the class at 200 yards using the same equipment during the final skills test. This was starting from standing (the low ready position) and having 6.5 seconds to make the shot at 200 (which I made from prone).
Gemtech's company reputation is the best out there. I don't own a Gemtech suppressor, but I have heard nothing but good things about them.
I own four AAC suppressor and am very satisfied with them.