Re: KAC NT4, AAC M4-2000, or Surefire?
I think it all depends on what you're trying to accomplish. Are you building a clone? Then the choice is already made for you (KAC NT4 currently, or Surefire SOCOM 556RC near future).
AAC makes a fine can, especially for the money. However, I somehow have never been bitten by the AAC bug. Lots of folks rave about them, but there are issues. While most AAC owners have never had a problem, there are plenty of people who have had trouble with their cans coming loose and getting an end-cap strike. This is why they went from the 18-tooth design, to the 51-tooth design, to the 91-tooth design. The mounts are not interchangeable, or backwards compatible.
The Surefire has the best mounting system, I think, especially when it comes to POI shift. The new design doesn't foul up the mount as much either. It is possible to mount the newer SOCOM can on the old mount and have it remain concentric to the bore, but rotationally it will not lock up like a dedicated mount. As posted above, if going on a SBR with the Surefire, I'd go with the brake. Not because I needed recoil reduction, but because the brake would act as the 1st baffle in the can, and can be replaced when worn away by the extra blast from the SBR.
The KAC is a war-horse. Literally. It's been around forever. Not the newest design or technology. It is tough, built like a tank, and it's heavy. But not by much though. Compared to the Surefire with mount, the KAC with mount is only about 4 oz more. The bore is slightly larger, so you should never have to worry about an end-cap strike. The can will not come loose unless you neglect to close the ring and leave the gate up. Yes, they are a little louder, mostly due the the larger bore. They tend to have a lower pitch tone to them, which some people find more pleasing.
So, in summary:
1. if money is the main concern, AAC is the best bang for the buck
2. if POI shift and latest edition for clone purposes is your thing, then look at the Surefire.
3. if you are after an accurate clone of current issue or past issue weapons, and durability is a major concern, then go with the KAC.
I went with the classic Mk18 mod 0 clone