Got one of the RRA lowers on 9/10/01(day before 9/11) thru Jim_Dandy with a few hassles from the FFL who did transfer('nother story) and one of my Xmas presents was a RRA lower parts kit. Just got RRA M4 tactical upper with pressed on fake suppressor. Got the upper from SableCo.
To my disappointment the upper receiver had some kind of a problem with the finish. It looked like the cement my sidewalk is made of. No shit. SableCo doesn't accept returns so I called Rock River Arms and talked to one of their gunsmith's, Terry. He said ship it and he'd take care of it. I did and he did. They replaced the upper receiver and had it back in less than a week.
Put upper on---very tight fit. After a couple of days together I'd say it fits perfect. Really like the flat-top detach carry handle. Very nice fit to all parts. Finish of upper matches lower perfectly.
Put in a couple mags. Thermolds were very tight when working bolt(they dropped free fine, even full). More so than my Colt or PWA(that one is tight in the magwell, I've gotta pull 'em out). USGI's: perfect.
Bolt wouldn't lock back, though. Eventually realized the bolt catch was just a little rough in the slot it rotated in and as such it provided to much resistance against the mag springs. Working this back and forth with alternating front and rear pressure solved that problem.
Shot great with no malfunctions in 50 rds. Seemed to be just about right on at 30 yd range. You could tell they had sighted the rifleand were very close to dead on. Hard to say for sure. There are so many damn bugs up here right now it got very hard to see. I'll range test it for groups when the bug hatch lets up. A few hassles but it will likely be my grab&go gun. Nice finish, excellent quality it appears. It is an excellent addition. I'd take it over Colt or Bushmaster, hands down. My more than 2 cents worth.