mac's right its a battery pack for halo/reloader/pulse(?)
ive had one in my halo and reloader for about 18months or so now....
it has saved me a small fortune in AAs already, i found its good for about 11 cases, could maybe squeeze another case out of it, it also cuts a shed load of weight fromthe hopper, you just have to put some foam/kitchen towel in the battery compartment with it (the rattling gets a tad annoying)