Barrel length doesn't matter. What carbine and mid-length are referring to is how far out on the barrel the gas port is. I believe a carbine is 7" and a mid-length is 9", but don't quote me on it.
"Breathe when you can, shoot when you should."
"Qui desiderat pacem, bellum praeparat; nemo provocare ne offendere audet quem intelliget superiorem esse pugnaturem"