Is this as simple as dropping in a 16" barrel into the current upper receiver group?  I assume the weapon wouldn't function properly just doing this.  Or does it require the whole URG to be replaced with a 16" one? (silly questions I'm sure)

(original DD MK18 5.56 pistol)

Thanks

Sure,  you could change out just the barrel to a 16" with carbine gas like the 10.3 on the Mk18 pistol. Or "easiest", get a complete rifle upper and slap it on.  Then either case you'd need to change buffer tube to one that uses a stock. But you could still go back and forth tween pistol and rifle, using the carbine buffer tube in place for both.

But cheap as lowers, complete uppers, complete rifles, actually everything AR is right now, makes more sense to me to just to make a separate AR rifle in whichever way best floats yer boat.

- OS

If you have a set screw or clamp on FSB or gas block, that makes it cheaper. Get a barrel for $70~ from Classic firearms, switch over everything; gas block, barrel nut, ect. Only thing you'll need is a flash hider washer.

Get a chinese CAR stock. I can buy them for $30 locally.

Bada bing!