I've only had this to the range about 4 times. The first time I had feed problems with both drum and stick magazines. After some maintenance on the magizines they fed better by the third time to the range. The last time the stick magazine worked fine. When I attached the drum magazine and fired the first shot the bolt latched open. I pulled back and released it and it chambered the bullet. The next time I fired it locked back again. I repeated the process about 14 times and gave up after my hand started hurting. Does anyone have any idea why the bolt would lock open. It's not even supposed to do that on the last shot.

Thanks in advance.