Originally Posted By: dlundblad
Can someone fill me in on the differences between a carbine barrel vs. one that isnt a carbine?
They are all carbine barrels, since AR-15's are technically carbines. What I think you are referring to is the length of the gas system. Carbine length has a 7.5" gas tube. Mid-length has a 9.5" gas tube.
The differences are not that much, but there are some benefits of the longer gas tube. For one, it increases dwell time, which is how long before the bolt unlocks and starts moving rearward as it ejects the spent cartridge case. This increases reliability with certain ammo configurations.
One other benefit of the mid-length is that since the dwell time is increased, it tends to lower the PSI at the gas port. The higher the pressure, the more wear and tear on the bolt, cam pin, and locking lugs. A mid-length system is thus likely more durable in the long run statistically.
Carbine: 34,000 PSI
Mid-Length: 27,000 PSI
The carbine gas system is an excellent system and has served the US military well BUT, I myself and many others think the mid-length system is better for the reasons stated above.
Carbine gas top, mid-length gas bottom
And here is a picture showing the other common gas length, rifle, compared to the other two systems above.