If your brake was installed by Bushmaster at the factory, it's probably blind-pinned and welded. You can remove it easily by locating where the blind pin was installed and drilling or dremeling the pin out, or cutting the brake with your dremel and cracking it apart with a chisel.
If it was silver soldered, you might be able to heat it up and unscrew it, but if you can't get it hot enough to melt the solder and take it off, you'll have to turn it off with a lathe or cut the barrel itself.
Your new brake will need to be around 2.25" long or longer to bring the barrel out to 16" overall length with the thread overlap. The standard wilson brake's not long enough, and you'd have to get Kurt's "A3 tac-brake" which is extra long for 14.5" permanent installs.