What does it do?

reduces recoil, but make a louder report.

I think I ment muzzle brake.  I'm looking at starting to buy parts for a ar15.  right now I'm looking at a complete barrel assemble from bushmaster. Either the 11.5 w/ 5.5 muzzle brake or the 14.5 barrel.  They are the same price, just wondering whether the muzzle brake is worth getting?

If I were you I would seriously consider a fake flash hider if you just want "the look" and you are building a postban. I put a brake on mine, and it is noisy and obnoxious. Some are better than others. Kurt's Kustom Firearms has a great selection of fake flash hiders that look the part, as well as brakes that look like flash hiders. He can make about anything you want. I wish I had gone that route.

I would rather go back to a target crown/plain barrel on my postban AR if I had a choice, than have the brake. My preban w/flash hider sounds so much nicer, more of a "crack" sound than a "boom" like the post ban with the brake (Cav Comp).

Just me though, get some different opinions or fire a friend's AR that has a brake installed. I would avoid the set screw type, they tend to fly off if you don't keep on top of them.

The Mini-Y brake has a reputation for directing the blast back at the shooter, just FYI.

I recently bought an AR from Clark, and it came with a brake. After about a month, I sent it back and asked them to cut it off and re-crown the muzzle. I am much more satisfied with it now. The only possible use I personally have for a brake on a .223, is if I were shooting prairie dogs without a spotter, and had to see the bullet impact. Otherwise, the cost of the muzzle blast is not worth it to me. I suppose there are certain forms of competition where the slight time saved in recovery and target acquisition would make one worthwhile, but I'm just a plinker.  

Yes, exactly.  Nix the brake, don't need it.  The 11.5" barrel you are thinking of does not have a 5.5" brake, it's a flash hider and unless you have a pre-ban lower, you can not have it.  Even with the 14.5" barrel, unless your lower is pre-ban so you can use a flash hider, the brake has to be permanently attached and bring the barrel OAL to just over 16" to be safe.

Look at Rock River, they have 16" post ban M4 barrels chrome-lined with a pressed on Faux hider (all show, no function) for about $190.

you don't want the stock Bushmaster muzzle brakes -- way too loud!  you are just going to end up sending the barrel off to Kurt or whomever to get a proper brake.  meanwhile you are losing muzzle velocity with ANY barrel that is "extended" out to the minimum length of 16" via a brake.  as noted above, one way to work around this is to get a full 16" bbl and then put a faux flash hider on the end.  so you get the 16" of muzzle velocity and "the mil look" at the same time.

or do it the way i did, and keep it simple -- order a "Patrolman's Carbine" upper or barrel assembly from Bushmaster.  a full 16" long, blunt muzzle, no brake; less noise, maximum velocity, with minimum length and weight.  

PURA2B-16M4, complete upper:
[img]http://bushmaster.com/shopping/uppers/Images/pura2b16m4.jpg[/img]

or

P BBL-16M4A, barrel assembly only:
[img]http://bushmaster.com/shopping/barrel-assemblies/Images/abbl16m4a.jpg[/img]

regards,
ar-jedi

edited to add:
ps: don't get the A2 version upper pictured above.  skip to the A3 version (no carry handle, just the picattiny rail).  and don't bother with getting the removable carry handle -- with that $100 saved you can get an ARMS #40 back up iron sight (BUIS) and use that for your irons until you put a red dot (e.g. aimpoint) or scope on the rail.

Muzzle brakes, as a product of their effeciency, are LOUD.

If you intend to use this weapon in competition (CQB, Three-Gun or Tactical Rifle Matches), an effective muzzle brake is a necessity.  The time saved in recovery and target acquisition are HUGE.  If the gun is just a plinker, then forget the brake; most people get tired of the noise/blast when it isn't of any benefit to them.