These have carbine length bbls' of 16 inches correct?
Yes.
Does it move the front sight out farther over M4 style to offer a longer sight radius?
Yes, the front sight base is closer to the muzzle.
Does the gas tube then become longer to meet the FSB?
Yes. Also, longer handguards are required.
Does this put less stress on the weapon as compared to an M4 but more than a 20'?
Yes, it cuts gas port pressure by about 25%. And the carbine DOES operate more smoothly as a result. It feels more like a 20" rifle.
Does a bayo fit these?
Man, if someone is close enough to use a bayonet on, you shoulda shot him already. If you are out of ammo, well, no true ARFCOMMER would run out of ammo in a SHTF situation. Bayonets make good figting knives, but you have to take them off the rifle first. Duh!