Hmmmmmmmmmmmmm.
Well, I know 30round mags are legal, and if I remember correctly, At a Small Arms Show in York they were selling the twin round drum mags.
But, to be safe, just check at your local Police Department.
Best place to go is probably the state police or attorney general but as far as I know if ya had a magazine capable of holding 100 rounds it wouldn't be illegal. This ain't Kommiefornia, it's Pennsyltucky gosh darn it--the land of the minute men....Get your mind out of the gutter ladies