I am hoping someone comes up with an affordable, viable option to extend to the “magic” 16" required. Maybe I am seeing it wrong but, I am concerned with the muzzle end of the device shown from US Machine gun. I’d prefer an open muzzle as opposed to one closed down to approximately 9mm. Hard to tell from the photos but, is there a “barrel” inside the device? How (if at all) would this device effect accuracy?
KAK Industries has a variety of flash cans and retro flash hiders that might be a better option.
I apologize for the delay in reply…
It interesting that you mentioned KAK because I happened to have one of their 1/2 x 28, 6" muzzle devices. I thought that would have been a perfect fit (considering the AK-V barrel is reportedly 10.5" in length). The problem that I ran into that derailed that course of action for me is that upon dry-fitting the KAK to the AK-V and measuring the barrel, to overall length turns out to be 1/2" short of a 16" length.
That’s unfortunate predicament that I find myself in. 
One of the great attributes of the KAK device is that it comes with a pre-drilled hole for a pin & weld. I believe that the pre-drilled hole is 1/8" in diameter. They are quite nice and I had hoped that option would’ve worked out. 
wow. Have you used the rod down the barrel and then taken it out and measured the rod? Put it in the barrel until it stops, mark with tape, take out, measure.
I have the same thoughts as you regarding the barrel down the center of the USMG faux suppressor. It is rated for a .45 and I can honestly say that I wouldn’t worry about any bullets bouncing around in there before exiting the end of the device, but at the same time, I question if that could be a potential issue.
I had expected the USMG to be more like the KAK device and essentially be an open-ended tube (if for nothing else than to be a weight saving option), but the USMG is more like a beefy, smooth bore barrel extension chambered in .45.
Indeed. I started measuring with a bamboo dowel that I would place down the barrel on a closed bolt until it stopped and then measured the distance of both the barrel alone as well as the barrel with the KAK on the end.
Then I fabricated a measuring rod at one inch increments and 1/4" increments at “key distances” of 12"-13", 14"-15", 16"-17", and 18"-19" just to have an additional measurement.
I accept that the measurements could be off a wee bit, but not more that 1/16" at the most.
Well crap. I know KAK makes a 6" one but it is for 7.62 and has 5/8x24 threading. Shoot them an email and see if they have something in the pipe. They might be willing to do it since they are already tooled up to make them and with the ATF ruling (pending lawsuits) it might be a decent seller for them.
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This KAK is at the 6" length.
It’s rated for .556 & 9mm.
And true to their specifications, it measures in at approximately 6.15".
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Just for the fun of it, a side-by-side comparison of the inner diameter of the USMG (on the left) and the KAK device (on the right).
I have a few CNC lathes with sub spindles if no good option is found.
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Again, all of this conversation is meant to find safe solutions to the recent ruling regarding pistol braces. With that said, IF I were to go with having the USMG pinned & welded onto the barrel end of the AK-V, there is going to be some modifications to the USMG device such as having to drill a hole for an aluminum pin to be welded and I would also want a small space milled or drilled into the end to allow the retaining pin to release over the back end of the muzzle device. I certainly do not like how the pin is left open on the back side of the sight post for gunk to encroach in there much more easily.
Maybe it’s just OCD? 
Ah…thought it was 5.5".
Could you use this with the kak?
You can, but would require an additional pin and weld.
But you could weld together with kak and have only one pin in extension where there is more surface to blind pin until this bull gets overturned.
You cant just weld. Has to be a pin and weld per ATF guidlines.
Its got to be resilient to 1100° (maybe 1200°, trying to remember without looking it up) to be considered an actual part of the barrel iir the regulation right.
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March 31, 1998
BATF
Firearms Technology Branch
Washington, DC 20226
Greetings:
I had some questions about permanently attaching muzzle brakes
and barrel extensions to firearms.
As you are aware, some barrels, particularly for some semi-
automatic rifles, come with muzzle threads. However, it may be
necessary, depending on the other features of the rifle, to remove
those threads, in order to keep the firearm from being considered
a “semi-automatic assault weapon”. Additionally, some firearm
barrels come in lengths below 16 inches, and in order to be
installed on rifles without making the rifle subject to the
National Firearms Act, an extension may be permanently attached to
the end of the barrel, by muzzle threads, so as to make the barrel
length at least 16 inches.
I was wondering what methods of attachment to muzzle threads
are considered permanent enough so as to either extend the barrel
length, by use of an extension, or to remove the muzzle threads as
a feature of a semi-automatic rifle which might otherwise be
considered a “semiautomatic assault weapon”
In particular, I have heard that welding is acceptable. If you
can, please advise me of the method of welding, and the required
weld coverage. I have also heard that high temperature silver
solder is acceptable. If you can, please advise me as to the solder
alloy, and melting temperature that would be considered permanent.
I have also heard that some industrial adhesives are acceptable, in
particular a product called “Rocksett”. I would appreciate
confirmation as to which, if any, industrial adhesives have been
found to be acceptable.
Also, if there are any other methods which I have not
mentioned above, which have also been found to be acceptable
methods of permanently installing muzzle devices onto rifles, I
would appreciate it if you could advise me of what they are.
Sincerely,
DEPARTMENT OF THE TREASURY
Bureau of Alcohol, Tobacco and Firearms
Washington, D.C. 20226
JUN 18 1998 F:FPD:FTB:RAT
3311
Dear Mr. :
This refers to your letter of March 31, 1998, in which you ask
about permanently attaching a muzzle device to various firearms.
A muzzle device, such as a muzzle brake or barrel extension, which
is attached to a barrel by means of welding or high temperature
silver solder having a melting point of at least 1,100 degrees
Fahrenheit, is considered to be part of the barrel for purposes of
measurement. A seam weld extending at least one-half the
circumference of the barrel or four equidistant tack welds around
the circumference of the barrel are adequate for this purpose.
A firearm having a muzzle brake, cap, or barrel extension
permanently attached by those same methods to cover the threads on
a barrel, would not be considered to have a threaded muzzle.
Please note, however, that any muzzle device or barrel extension
which functions as a flash suppressor or grenade launcher would
still constitute one of the qualifying features of a semiautomatic
assault weapon as that term is defined in 18 U.S.C. section
921(a)(30(B). Industrial adhesive products are not an acceptable
method for permanently attaching a muzzle device.
Mr.
We trust that the foregoing has been responsive to your inquiry.
If you have further questions concerning this matter, please
contact us.
Sincerely yours,
[signed]
Edward M. Owen, Jr.
Chief, Firearms Technology Branch
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Thanks for that. So you can tack weld it in multiple spots or go in a half circle.
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But they can change the rules whenever they so desire…
This might help too.
https://hughesprecision.com/product/barrel-extension-threaded-adapter-1-1-2-long-by-1-2-28-tpi-to-1-2-28-tpi/
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