Complete factory assembled shotguns usually have parts well fit to each other. Shotguns in the hands of end users is where they become loose.
Though there are + or - tolerances allowed, I doubt the factory chooses to fit parts from opposite extremes of those tolerances.
30-40 years ago, parts were fit for reliability, by hand, were fit to be much smoother functioning. A bit looser too.
With MIM technology, or CNC machines, you don't need to fit parts any more. The process allows for much more consistant tolerances where the parts go from machine to final assembly, no craftsmen needed. ... and they're rough. ... and maybe that's good, because for a better fit, you can always remove metal, you can't put it back.
So we have parts from different factories, different Countries, different work ethics, with different tolerance allowances and different metals and different ... you get the jist of it? . ... and all these parts are expected to randomly interchange somehow. Isn't going to happen all the time.
Nothing wrong with the Grizzly as it is. Why flip and flop parts with a REM? Just keep them as they are.
I think people who buy the Norcs, really only want the short barrel I guess - and the price is certainly right on this one.

Not like a factory $300. Remington barrel that fits. Problem is, not everyone wants to flaunt a Norinco, but having just the barrel, well, you can't really see what it is on a REM receiver.
Whatever you buy, buy lots of ammo and open her up. MAKE IT LOUD!