I've tried a bunch of "match" loads through it, trying to get a decent group, but the best groups I've gotten have been around 2" at 100 yards. These same loads have been around .9" to 1 1/8" out of my LMT M4. I tried removing and reinstalling the barrel, and really snugging down the barrel nut. I tried supporting the handguard at the middle, and back toward the magazine well. I checked to make sure my rifle rest was solidly locked down and screwed together tight. Getting a really good group out of the ACR is proving to be frustrating. My best load to date through the LMT has been a round with a Midway 55gr varmint bullet, which the ACR puts into sloppy 3 inch groups. Maybe some of the lighter MatchKing or Hornady bullets will do a little better from the ACR, fingers crossed.
I'm not sure if the 1:7" twist barrels are available yet. They have been extremely slow introducing barrels for the civilian market, which kind of makes the "A" for adaptive kind of a joke. Mine is a 1:9" twist. I was told that 1:9" twist barrels will sometimes shoot heavy bullets just fine, so I tried some. The Nosler 77gr Custom Competition bullets just sort of sprayed. I actually couldn't keep them all on a 14x14" target. It was really that bad. 75gr Hornady match bullets did very much better, but still didn't produce anything resembling an impressive group. 69 grain MatchKings were good, but still not impressive.
But at least it runs. It's chewed through the softest shooting ammo that I could find, and obviously has no problem with hot 5.56mm spec ammo. Steel case ammo has not yet posed a problem, at least not with the ACR.
It does have magazine preferances, but then most rifles do, to some extent. A Fusil steel magazine or two have seemed to lock in, and pass the pull test, only to fall back out when a shot is fired (embarrasing). A couple of my old English SA80 steel magazines lock in well, but then you have to really mash the magazine release to get them back out again. But the ACR chugs along happily with G.I. aluminum magazines, PMags of various generations and capacities, TangoDown, C Products stainless steel, Lancer and Troy magazines. I'm not sure if Fusil is even still making magazines, and I suspect that a lot of the old English steel SA80 magazines have been crushed, scrapped, or trashed, as 3 of the 5 that I bought in 2002 have proven to be sub-par.
Oh, and I dumped the trigger that came with the rifle. I bought a Geissele SSA-Enhanced AR-15 trigger on sale from Palmetto State Armory, and installed it with the ACR hammer spring, which doubles as the selector detent spring. Soooo much better. I run an SSA in my main AR-15, and I've got another with a 15-minute trigger job. The ACR trigger really sucked compared to both, and couldn't even compete with a Tapco AK trigger. I usually take 2 rifles with me when I got to shoot, and the ACR always came in 2nd place because the trigger pull was always heavier than whatever else I took. Just unpleasantly heavy if you have anything to compare it to.
Okay, so maybe it wasn't so quick an update.