No one will sell a under 16 inch barrel with a screw on flash suppressor not matter the length of the suppressor as it is not consider permanent....
Kinda' mostly correct, and
for our purposes here:
100%.
Reality is... you can indeed buy and own a barrel of less than 16" length, but you will be heading for felony land
if you have "access, possession or control" of any AR15 lower receiver. You can fight that out in court, if you so wish. This same rule applies to many m16 parts or even something like a Glock stock. You can argue your case on this court of the internet, or you can simply avoid these sticky areas in real life.
The 16" barrel lengths supplied by every civilian grade manufacturer are actually 16.25". The ATF defines a permanently affixed flash hider (or
whatever thing you hang off the front end, I'll just refer to it as a suppressor) as that it must be "blind pinned and soldered". This is generally taken to mean that a steel pin is inserted into a hole drilled through the suppressor and half way into the barrel right at the threaded area. A steel pin is shoved in and it is welded over (misnomered as soldered), ground smooth and finish (blind) is applied. It would take an x-ray to determine this and such delicate technology is usually applied by the ATF
after they test your cool toy by trying to twist your barrel & suppressor apart using a 12" vise and a wrench the size of an oar. Whoopsie for you. Sorry about that.
The length of the barrel also takes into account the "barrel extension", this is the part of the barrel that most people view as the "chamber". For ATF purposes, it is permanent enough to be considered to be part of the barrel, at least on that big hole end~ that's where the tape measure is applied.
Many years ago, when the 11.5" barrels on M16's couldn't be made to function reliably, Uncle Green figured out that spinning a 5.5" flash suppressor (hollow tube) onto the end created enough vortex suction to make the weapon cycle on full auto. This begat the XM-177 carbine.
In the spirit of "function follows form", many buyers today specify this odd XM combination (with the flash suppressor
permanently attached) for their AR15 application. There is no reason other than chicks will dig it. You have all the benefits of 11.5" of rifling and a 3/4" tube hanging out front making it 16" overall. A couple of steps past
needing the m4 configuration over a basic CAR. Whatever.
Several civilian parts houses offer 14.5" barrels with permanently attached (per ATF) suppressors already installed, giving you an OAL of 16.25". They usually do this with an "a2 looking" flash suppressor that is made just a wee bit longer than milspec. Some other outfits will do the machine work and install whatever you want, depending upon the depth of your wallet. Or, you can DIY, but be careful of the UPS man handing you a package anywhere near (at the house, county or nation where you have access to) your ar15.
Ok, you have one all installed per ATF spec... The problem that arises is that most any barrel device such as a gas block is designed to fit over the .75" maximum diameter barrel, which you will now noticed has that larger diameter flash suppressor in front of it. All is not lost however- there are a number of gas blocks which are two piece, and assemble clam-shell fashion over the barrel- not needing to be slipped over the .75" muzzle.
Stick with the 16" unless you have the paper.
