Excellent shotguns, IMO. I enjoyed the ones I have had, and found them to be reliable and fast. You can use many Mossberg parts on them, because IIRC these are essentially Mossberg 500 on the inside. I know forend with interchange as will stocks, though the receiver will over hang a bit.
Their popularity seems to be on the rise, though I dont know why. IMHO, its a no more than $250 in great shape gun, but on GB, some have exceeded that by almost $100. I fell for them because of Steve McQueen's Carter "Doc" McCoy work with it in The Getaway.
I prefer the K120 to the K1200. The later, towards the end of production, ran into heat treating issues.
Note to "That Guy": my K-120 has the exact same sling swivel arrangement, it looks like a factory setup.
Little known fact: This was designed by the guy who actually did the bulk of the M-1 carbine design (who wasn't that dude Williams). It sort of combines elements of the Remington 31 and Winchester 12 in a cheaper to make package. Although, comparing it to modern designs, it looks like the height of old world craftsmanship. I suspect, but do not know, that the Mossberg is a further streamlining of the design for even cheaper production and to include dual action bars and interchangeable barrels. They certainly share some elements but I would be surprised if any internals interchanged.
Before my time but I don't recall reading anything bad about them. I missed buying a RS K-120 on GunBroker a few years ago and I still kick myself for that. I've wanted one since 1972, and here's why...
Start the video at 1:25.
"Gunfighting is a thinking man's game. So we might want to bring thinking back into it."-MDFA
Beware of my temper, and the dog that I've found...