Hmm... I was skirmishing yesterday, and my battery inevitably lost charge. So, given that I don't have but one battery, I asked my buddy for one of his (he has a Cybergun Thompson M1A1). Upon insertion of the battery, I noted that it didn't operate. I tried another. It, too, didn't work. I eventually put my MP40 up (sad face...) and grabbed my MP001. Upon recharging my original battery, it went back to shooting like a dream. What is this black magic?! They're all the same, 8.4 Volt, 1100 mAH. Any theories on why this particular brand of batteries didn't work? I'm not rightly sure what brand these batteries were, just that they had no name and were wrapped in black plastic instead of the usual transparent plastic. I'm afraid of buying new batteries for my MP40 now. I don't want to drop 50+ bucks for a battery that turns out to be incompatible.