Or is it sorta popular only because it’s about as short as you can go on a Pinned and welded job?

No, that's about it.

For those who don't want/can't have a SBR, this gets them the shortest possible barrel (and dwell time) for their suppressor host AR that is still 16" with a pinned and welded suppressor mount muzzle device.

If you use the uber long suppressor mounts like the Surefire Socom, Keymo, Silencerco ASR, etc., then 13.7/13.9" will get you the shortest possible, when P&W, and that's about it. Sure it might save an ounce or two, but then again the barrel profile can vary so much between vendors, you could end up with a heavier profile 13.7/13.9" than a more commonly available 14.5". So that's kind of a wash. Could that max .8" make that much of a difference? I think not. If you really want shorter than a 14.5" go with 11.5/12.5", if possible. You'll likely have more options of profiles/gas systems/and general availability with 14.5". Also, since 14.5" is yesterday's news, you'll likely get better pricing/availability with them.

Now that Plan B/Xeno are taking over, 13.7/13.9" will likely be even less relevant as Plan B has a hard enough time getting 14.5" barrels up to 16.1". The new mounts are shorter and lighter than the ancient 2.5-3" mounts.

I think Noveske may have originated that barrel length some years back with their 13.7 inch "Infidel" barrel. Adding their (pinned and welded) KX3 flash suppressor / forward blast director brought the overall length to exactly 16 inches. I had one of these back in, I think, the early 2000s.

NOPE

For me 11.5, 14.5 0r 18.

with $0 tax stamps it would seem dead except in states the have no SBR's. What is the point of a couple of inches of fh steel rather than actual barrel?

quote:

Originally posted by hrcjon:
What is the point of a couple of inches of fh steel rather than actual barrel?

Shorter OAL.

13.9" + 2.1" suppressor mount muzzle device = 16"
13.9" + 6" suppressor on mount = 19.9"

16" + 2.1" suppressor mount muzzle device = 18.1"
16" + 6" suppressor on mount = 22"

I think 13.x" pin & weld can be a trap. If the goal is to fit everything to a legal 16", I think a 14.5" barrel and less flash hider are better.

If we are talking about a suppressor host 13.x" muzzle devices limit options for suppressors to only those with long blast chambers (Surefire, Dead Air and a few others). Some of the flash hiders for 13.x" like the SilencerCo p/w for ASR push the suppressor mount threads forward, so you are effectively welding 1/2" of barrel back onto your barrel.

If you go to a 14.5" P&W you get a lot more options in suppressor, you jump from about 10 suppressor options to way over 50.

Now if it's a 13.x" SBR some might consider it no mans land, but I can see a point to it. You can fit a mid-length gas system in a relatively short and light overall rifle. The K size suppressors are getting better and there's a trend in shorter blast chambers. For example the 4.5" CAT KK blast chamber is so short it would not work with a P&W unless you had a 15.1" barrel. I would build a 13.7" SBR with a CAT Nano flash hider (1.44" overall length, only adds .84" to the rifle) and a 4.5" CAT KK suppressor. The sound suppression is the same level as a Surefire RC2 according to Pew Science, but the overall setup would be 3" shorter.