What is the optimum barrel length on a SBR/Pistol set up?
Mike/GarandThumb has a few thoughts on the topic. Despite the proliferation of 10.5″ Mk18 semi-clones there are upsides and downsides to each barrel length.
14.5″ is a very good length for an SBR, but you aren’t changing very much from a conventional rifle or even legally leaving conventional rifle status depending on configuration/permanent muzzle device.
10.5″ is a more maneuverable length but you are losing velocity and changing up your gas systems.
8.5″ is a incredibly compact size and has the greatest advantages on that front but it trades the most on velocity and several other compromises.
I own 20″, 16″, 14.5″, 10.5″, and 8.5″. All are accurate 5.56 carbines. The one that gets picked has largely to do with which aspect is most relevant to the job at hand (muzzle velocity, optics, suppressor compatibility, etc.)