The word from the BATF is use of a 20 round drum with a Saiga 12 will cause the combination of the two to be ruled a NFA destructive device.
This will require a $200 tax stamp (NFA federal paperwork) in addition to what Mike wants as a price for his drum. Any further questions about this legality contact the BATF Tech Branch.
ATF
Firearms & Explosives Import Branch Attention:Chief Sterling Nixon
244 Needy Road
Martinsburg, West Virigina 25405
The new phone number is: (304) 616-4550
The new fax number is: (304) 616-4551
Hours: 8 a.m. to 5 p.m. Monday through Friday