No "M4" has a 4" FS.

Those are for guys trying to recreate the look of one of the earliest AR-based carbines: the XM-177E2 of the Vietnam era.

[img]www.usmc.mil/images.nsf/b7a610ddc1be59598525650500718300/C7D789DDF9EF4B20852566D400020083/$FILE/xm177e22.jpg[/img]

[img]darkstar-sifi.co.uk/images/bbguns/aeg/sae32.jpg[/img]

There's a couple of problems, though.  First, the barrel, even with a permanently-attached FS, isn't 16", so the "civilian" version uses a 5.5" FS on an 11.5" barrel.  Second, the originals used a device that also somewhat reduced the sound signature.  ATF ruled these devices to be sound suppressors, even though they don't reduce sound levels to anywhere near the degree that a normal suppressor would.

Here's a pic of Bushmaster's non-NFA "pre-ban" XM-177E2-style upper:

[img]www.bushmaster.com/shopping/uppers/Images/bura2b115555.jpg[/img]

-Troy

[:D]

For those of you scratching your heads instead of laughing at Chuck's "velocity" references, he's alluding to the fact that the velocity/fragmentation/wound potential relationship as it related to M193 (and other "not-necessarily-intended-to-fragment-or-expand") ammo was not understood until the 80s, when Dr. Fackler released his studies to the public.

Thus, no one really thought about or cared about the loss in velocity by chopping the barrel so short.  It was just *assumed* that the [b]carbine[/b] (Chuck, a submachine gun, by definition, shoots pistol-caliber ammo, regardless of the term the Army uses for the XM177s) would do its job.  It wasn't until 20 years later did we understand just how much we had reduced its effective range by using barrels so short.  Had we been using M855 ammo back then, it would have been a disaster.

-Troy