In most rifles, it only requires swapping the bolt, something a qualified person can do in seconds, in others, the upper receiver, maybe a 1-2 minute job no harder than proper cleaning. Pistol conversion kits are similarly simple.
Don't be fooled that it's some long, difficult process so unlikely for that reason, it's simplistic and fast....and the conversion is just as easily and quickly reversible.

Edit: That didn't sound like an 40mms I've heard, more like a 9mm pistol with a light load. Any kind of 40mm round at that range would be brutal

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There are conversion kits to make a Glock 17 automatic. It's my guess that's what he's using as Glock 18s are comparatively rare and FPS Russia doesn't seem the type to make that kind of naming mistake.
>> ^NaMeCaF:

A fully automatic Glock 17... isn't that a Glock 18 then?

>> ^xxovercastxx:
Stop the presses; ant has posted a video about strap-ons.