This is the GSG 110 round .22LR rotary drum magazine for the GSG 16, also fits older GSG 522 and GSG 5 rifles.
It's quite a piece of kit! Offering the chance to blaze through 2.2 boxes of .22 ammunition without reloading! The drums constant spring tension is easily taken off during loading using handy finger holds on the drum.
Please note: GSG recommend non lubricated round nosed ammunition. Please read the instructions before use.