The good, the bad and the completely absurd...
The Chinese SKS was deemed by ATF to be a non-sporting firearm back in 1989,
if it had bayonet attached. All Chinese SKSs imported after that time had the bayonet removed. Any Chinese SKS that was already in the US
and had a bayonet attached was grandfathered.
There is a little law designated 922(r) which says you cannot assemble from imported parts a firearm that is not legal for importation.
ATF has ruled that regardless of when the rifle was imported, if it does not have a bayonet attached
now, installing the bayonet
now would be in violation of 922(r).
Now pay close attention.
The
act of
attaching the bayonet is illegal. It is
not illegal to be in possession of a SKS with bayonet in violation of 922(r). At most, the rifle would be classified as contriband and seized.
Remember however, that I am not a lawyer and do not play one on TV. To the best of my knowledge, no one has ever been charged with putting/having a bayonet on a Chinese SKS what should not have one.