12.5" is a great length. I have 2 Noveske Crusader 12.5" builds and they are handy and function reliably. Many years ago I did a ton of research on this. From what I remember, pressure drops off pretty significantly around 12.5" for the standard 55gr XM193 round. I can't seem to find the graph online. Any shorter, you get minimal dwell time, a lot more unburned powder at the muzzle resulting in decreased velocity, a huge fireball, super loud, etc. I can't remember who it was that wrote the article but I think it was on DefenseReview.com? Pat McNamara or maybe Kyle Defoor? Any way I decided that 12.5" was the right length for me. I also had a GemTech suppressor that specifically said in the instructions not to use it on a barrel less than 12.5" so that was the clincher. These days I don't think I'd even bother with the SBR thing; I'd just get a 14.5" with a pinned/welded flash hider and call it a day. Way too much hassle to save a couple inches, and we need to quit supporting this NFA nonsense IMHO.