What length barrel should I go for on a 9mm AR SBR? My local shop has two in stock, I'm going for one of them. One is a 7" barrel the other is a 10.5 inch barrel. I like the look of the 10.5 better, looks more balanced and more AR like to me, the store recommends the 7". Which is the way to go? What pros and cons of each length when using 9mm ammo, will also be suppressed. Also, what is the NFA rules on going with a shorter barrel than what is initially registered? Since both of these are factory SBRs, they're already registered as 9mm in the length they have. If I buy the 10.5" one, would I be allowed to later put a shorter .223 upper on that lower and still be legal? Would it require paperwork? How hard is that to get done? Would I be better off just registering one of my other lowers as multi caliber and with something silly like a 5" barrel to be safe? I like the idea of an off the shelf SBR since its just marked "SBR" on the lower and dont need to engrave anything else on it. Thanks.