Is the only difference between an M4 upper and a mid-length upper the position of the fsb? Are they both 16 in.?
Yes the only difference is the position of the FSB. They are a 16" barrel and the gas port is still in the regular position, but covered by the handguards. The only advantage to the midlength sysytems like the Bushmaster Dissapator is the longer sight radius.
I would add that another benefit is the longer gas tube, less wear and tear on the bolt from reduced pressures. Donnie
I would also submit that, Shorter gas tube also reduces the gas pulse timing so elevated pressures are not a huge issue. Sight radius isn't an issue with the M-4 since the primary mode of sight is with optics now, not 'iron' sights. Also, not all M-4 weapons have a 16" barrel, some special application rifles are shorter, depending on application... M-4 is simply the military designation for the carbine version of the M-16 variant. *Shorter than 16" in the civilian world is a violation of federal law with out proper BATF paperwork.
Nope, the 'M4' most certainly have the indentation on the barrel for an M203, 40MM grenade launcher. The early 'Carbines' didn't, but the M-4 has.
