For those new to rifles , carbines , pistols , and SBR’s (Short Barreled Rifles) . 1)Overall length 2)Barrel Length 3) Use of stock for carbines/rifles or brace for pistols ; there’s other things to consider like pinned & welded muzzle devices to make your pistol length barrel a 16” + barrel to use a stock of your choice on. I’ve seen a few posts lately (mostly new members or so called) trying to sell SBR’s listed as “AR-15” and I’m sure most Folks on here know how to determine barrel length? If not Brownell’s.com offers a very easy and straightforward video. Please , “Ignorance is no exception to the law” …. that’s what you’ll be told for making an honest mistake. (Constitutional or not), we have NFA guidelines right now that the ATF will use to take your firearms and liberty. Before I got into firearms, I had no idea how many commie rulz we had against firearms that make absolutely no sense to a new gun owner, in my experiences. JUST A HEADS UP. Have a good one.