I got this because I wanted to be able to hide my battery in the stock tube of my m4. Fit's beautifully. however, the battery was junk, after a full charge it couldn't even pull the piston back once fully, got half way and stopped. at first thought maybe my gun was getting jammed or something, but hooked up a regular 8.4v and fired fine.
maybe I just got a bad one, but I'm not gonna pay stupid shipping to get a replacement and I'm not going to buy another one to see if I get luckier the second time.