[L473ncy]

Maybe Sub-C sized cells, either that or something like LR23?

Also +1 to using a Lipo. A 7.4V should usually do the trick and if it doesn't have enough power to pull the spring back an 11.1V will do it if not a springchange and/or high torque setup. If you want to go LiFePO4 then 9.9V usually performs more or less equivalent to a 10.8 or 9.6 (kind of depends on temps and how much juice is in the battery), the 9.9 is also easier on internals due to it's inherent design/chemical properties and subsequently has something like 15C discharge rating.