Posted on: 2010-12-14 20:46:00 -0500I’ve been debating between the AAC SPR/M4 (now discontinued) and M4-2000 suppressors for use on my AR15s. One consideration has been mounting them on a short-barreled rifle (SBR). The two mount quite differently. The SPR/M4 is 8.625" long and adds 4.9" to the rifle beyond a standard A2 flash hider. This is due to the mount sliding back over the barrel to provide an expansion chamber behind the muzzle. This large expansion chamber apparently helps reduce blowback. The SPR/M4 requires 2.49" of exposed barrel behind the muzzle. This means that it will not work with a 10.5" barrel and requires a short gas block on a 11.5" one. A 12.5" barrel is the shortest standard-length barrel that will work with a normal gas block, but a custom 11.75" barrel would also work. The M4-2000 is 6.625" long and adds 5.1" to the rifle beyond a standard A2 flash hider. Its initial expansion chamber is much smaller than the one in the SPR/M4. As a result, it mounts just fine on a 10.5" barrel. One consideration is baffle wear on an SBR. AAC makes a muzzle-brake mount for the M4-2000 but not for the SPR/M4. The brake would provide a sacrificial baffle that’s much less expensive to replace than the suppressor. I’m currently kicking around an idea like this to cobble together an SPR/M4 mount with a muzzle brake, but AAC offers a lifetime warranty on their cans, so it’s probably nothing to worry about.