Started to thread an early Armalite AR10 16" SS barrel, and discovered the first inch has a false muzzle "cap"..
The first .800" of the barrel is reduced to a smooth stub about .550" diameter, and a matching stainless sleeve was slipped over this, and turned to match the barrel.
I wouldn't have ever noticed it except for the threading.

I am suspicious that one of the earlier previous owners, who is a "machinist in progress" and has all the equipment, may have attempted to thread the muzzle, botched it,
and then did this to cover it up before he sold it.  
But, I don't know what the factory might have done, and am have no way of proving who might have done this, so am asking here if anyone has experience with this.

Is this something that could have been done at the factory?  (can't imagine why)

Thanks

Maybe made ban compliant during the dark period?

Pics?

Armalite did have some MB pressed on during the ban as well as the "nubbin" years where they had  an area prepped to thread but no threads from the factory.

I considered the muzzle thread ban period, and this as a possible fix.
If they turned off the threads, this could have been a cosmetic cap.  
Will call Armalite next week and will post if I can get a definitive answer.  

Just took some measurements.
Fortunately, there is enough full diameter barrel to cut and rethread; a pinned and welded suppressor mount will stay over 16".

And the answer from Armalite is:
This was news to the current workers at Armalite.  First time they've seen one come apart in this way.
Best speculation (and I agree) is that it was a fix to use the threaded muzzles in inventory during the 94-2004 Clinton plague.

Funny thing; when I tell young shooters that standard capacity mags and muzzle threads were banned at one time, they think I'm pulling their leg.  Like the internet -it's always been this way.
I don't even try to tell them that from 68-86, you had to register every purchase of all ammunition, even a single round of .22 Rimfire, primers, and empty cartridge cases.  
They'd really think I've lost it.

What state were you in that required registering all the ammo and components?   I was in Oklahoma at the time and never had anything like that.

Greg