I'm looking for a handguard for my Ar, but the carbine length are too short, and the mid-length are too long. The one I have is 8 5\8" lip to lip, and 8 1\8 surface. I can't find anything that length. All these so called "drop in" handguards ar too long or short. So what's the deal? Do I need to modify the gas system to change my handguard?