Where did you get it's 4 parts to be 922r compliant? It's 6 parts. BuildMP5VerifyCompliance < Gunwiki < TWiki
Mag - 3 parts
Stock - 1 part
Handguard - 1 part
Vertical grip - 1 part
I would check first with HK for parts, then RTG, [insert any internet shop], if all else fails and you want to get financially rape buy from HK Parts.