I have two 75 round drum mags for my Norinco Type 86s Bullpup AK. I've never had any trouble from them. There are 2 main types, Chinese (Norinco) and Eastern block. Both of mine are Chinese so I can't help you on the Eastern block design ( It's different ). The front of the drum opens and you drop the rounds into the drum, close the cover and the wind the tension spring. The magazine can be stored loaded and then wound when needed. This is a nice feature to prevent posible damage to the spring from long term, loaded storage. You can read more about it at http://www.ak-47.net/ak47/drum.html
I had some trouble with mine at first.
I was used to the MWG 90rder which loads like a normal 30 rd mag.
But have since found the 75 to be very good.
I like the load and wind when used feature of drum.