Drums are not the best choice for a semi automatic rifle. First off drums are extremely expensive when compared to magazines. A quality drum, like a MAGPUL variant costs around 120 and holds sixty rounds, whereas a 30 round Magpul Pmag costs between 10 to 14 dollars.
Drums are often unreliable as well, and are known to be plagued with problems. There is a very real reason we have never seen drums in widespread military use. Their higher capacity advantages is not worth their price, the amount of space they take up, and the necessary maintenance required to make them run.