There are dozens of legal drums on the market; why do you think he had an "illegal" one?
Or, is that just typical gun ignorance talking...?
“Or, is that just typical gun ignorance talking...?”
No, it’s atypical gun ignorance. I own plenty of guns, but not an AR. My son, however, does own one. A little research on my part found that yes, there are many ammo drums available for an AR.
Now since you are an expert, please tell me with complete confidence that they will handle the feed rate of a full auto AR rather than the semi-automatics that they were made for without modification.