I came across this info on the "internets":
"The mid-length system is approximately half way between the carbine and rifle length gas systems, while maintaining the same barrel length as a carbine weapon. It was created for various reasons, primarily to increase gas pressure in an attempt to increase reliability of the carbine weapon. The U.S. military M193 and M855 spec ammunition does not generate enough pressure to cycle the carbine length system with 100% reliability in certain adverse conditions (this has been solved with high pressure loads like Mk262). So, the gas system was lengthened slightly on a carbine length weapon to generate more pressure to ensure reliable cycling."
Gas pressure is decreased.... Correct?

