A few things to think about if you are about to build an AR-15:
Are you building a rifle because you want to shoot in 3-Gun matches with it?
Are you building a "BBQ Gun" that you will ONLY bring out to show your buddies when you are waiting for the ribs to finish smoking?
Are you building a rifle for the SHTF moment when the zombies start swarming your yard?
Are you building something that might be used to shoot hogs ?
Just my two cents but I don't see the utility of pinning a barrel on a "work" gun. It doesnt matter if it is for hogs or zombies, pinning an oversized muzzle device on a barrel just to "make it legal" is something that you should only do to a BBQ-Gun.
-If you plan on taking it out and using it for 3-Gun - stick with a 16 or 18 inch barrel (my opinion of course)
-If you are building a BBQ gun - why stop at 13.7 - just build a "pistol" with an "arm brace" then you could put a 7.5 or a 10.5 or any other "pistol" barrel and still not have to worry about SBR problems.
-If it is a SHTF gun, a 16 is short enough to maneuver easily but long enough to accurately engage bad guys several football fields away.
-If you are building a hog gun a 16-18 would be just fine
An AR-15 is one of the most versatile platforms on the planet. If you want to shoot targets at long range, toss on a 6.5 Grendel upper and away you go. You need something to shoot BIG hogs - find an upper in 300 Blackout or 458 SOCOM and have at it. If you just want a tactical rifle with anodized M-LOK hand guards and wicked cool flash suppressors - there are more vendors than you can shake a stick at that will give you the coolest looking gun since Han Solo popped Greedo with his custom DL-44
...and because of the AR's inherent versatility, with one well built lower receiver, you can have a BBQ gun, a target rifle, a SHTF rifle, AND a Hog Killer for just a fraction of the cost that you would spend on four or five special purpose rifles.