It's no trouble, it's a valid question. Unfortunately, each and every 922(r) case is usually something that needs a determination by ATF because they often choose to selectively enforce laws. I agree with Adam above, 922(r) isn't something they go after you for, it's something they add on when they find an illegally modified weapon at a drug bust, or when busting a cockfighting ring.
Here's my personal interpretation of 922(r) in your situation, take it with a grain of salt:
922(r) basically says you can't manufacture,
from imported parts, any weapon which you couldn't otherwise legally import. Current law states that it is illegal to import
any semi-auto, non-sporting (i.e. military surplus) firearm that
is not a C&R. The recent Sino-banians got in because, and only because, they got a C&R classification. When you modify these types of firearms, you can lose C&R status, basically invoking 922(r).
Things you can change w/o invalidating the C&R status:
- Swapping to another wooden stock of the same type (i.e. not a thumbhole or Monte Carlo)
- Replacing a damaged or mismatching serialized part with a replacement part identical in form & function. (10 round box mag with another 10 round box mag)
- Adding a spring loaded firing pin.
- Putting on or taking off the bayonet to shoot, store, etc.
- Adding a scope, using different sights, slings, etc. (The way I understand it, these are temporary shooting aids that can be easily removed)
The things that alter the original military configuration & cause the firearm to not be considered a C&R:
- Adding a composite stock, or a wooden stock in a thumbhole or Monte Carlo configuration.
- Adding a composite or metal handguard.
- Adding a detachable magazine.
- Cutting the barrel down to 16.5".
- Permanently modifyting the bolt carrier for LH operation.
In your case the cheese grater and detachable mag would revoke C&R status (assuming it had C&R status to begin with) and invoke 922(r) and you'd have to swap out two additional parts (assuming the mag and hand guard are US made) to be 'legal'
So what happens if you have a late Chinese that is not a C&R? That's a darned good question. Since 922(r) only covered the assembly, one assumes that since the gun is already imported into the US, it's legal in the condition you received it. If you begin to upgrade
it in any way, I think they could argue that's a new 'assembly' of it and 922(r) would apply to even simple parts upgrades.
It's a confusing minefield. They like it that way, as confusing as possible means that they can drop the hammer for seemingly insignificant things whenever they want to.