Carbine Vs Mid Lenght Gas System...

The military M4 design is a carbine-length gas system with a 14.5" barrel. The civilian version keeps the same gas system length but extends the barrel to 16", adding 1 and 1/2 inches of dwell time and therefore more gas pressure. Most 16" barreled, carbine-length gas system AR-15s are over-gassed; at best an over-gassed rifle will have a sharper recoil and will violently throw brass. At worst, an over-gassed AR will attempt to extract the fired case while the bullet is still in the barrel and the case will be ripped from the chamber while it's still expanded against the chamber walls. This can cause everything from a little flash out of the ejection port to chewed-up brass and failures to extract.

A heavier buffer will fix the problems caused by an over-gassed carbine-length 16" barreled rifle, but a mid-length barrel lowers gas pressure and minimizes those problems to begin with. In my opinion, a mid-length gas system is a superior design for a 16" barrel 5.56 AR than a carbine-length system.

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