The only reason the M4 Carbine has a 14.5" barrel is simply so it will mount the standard M7 bayonet. And so it could use standard M177E2 handguards (AKA "carbine") handguards. The M4 Carbine was a Colt development for the Sheikdom of Abu Daube years ago.
16" barrels in "mid length" configuartion will mount the M7 as well.
-- Chuck
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