My 7.5 inch had a horrible muzzle blast issue. It was like getting blasted in the eyes with a camera flash from 3 feet away. Once I put a Kak flash can on it, it became a pleasure to shoot. My advice about barrel length is to decide what size suppressor you might eventually end up getting, and whether or you want to be able to extend a handguard out over it. In that case, the shorter the better...
Exact advice I got from a friend at Freds. I have several 16'ers and wanted something shorter. I've got most of it finished and waiting on the brace and orward blast diffuser. Hopefully I'll get to shoot it this weekend.