The muzzlebrake is coming off with extreme prejudice.  I was thinking of A2 FS because it has a fully enclosed bottom.  Any drawbacks? I'm looking for the most noise reduction (compared to the ridiculously noisy Colt M4 brake).

Thanks,
bd

only supressors can reduce noise

muzzle brakes INCREASE the normal noise of the normal bbl, depending on ammo and length

A2 is the best b/w the 2 of the listed

i'd go w/ a phantom myself; its in the middle b/w the vortx and a2 on flash reducing

The A2 Flash hider will have to be indexed so it is a little harder to install.  You will need peel or crush washer.  Whoever cuts off or removes the muzzle break and threads the barrel can do it.  The A1 is simple you just screw it on.  The A2 offers several upgrades including the bottom being closed so it will not kick up dust when prone and offer some muzzle rise control since the blast goes up and sideways but not down.

A2 is the best b/w the 2 of the listed

How do you figure that?  

I use both, the A1 is slightly more effective with FS, but the A2 is going to reduce dust signature when firing prone (but will still show).  Its a toss up for what you like better.

As posted above, the Phantom or Vortex are much better for reducing muzzle flash.