Get an Intellect 9.6V pack. These are currently the best cells on the market with a 1200 mAh capacity and a ridiculously low internal resistance. It will crank over your gun no problem, and if you can fit a 9.6V 600 mAh in there you can fit this one too. Best of all, despite these amazing specs they are CHEAP! How's about one for $18.95 USD?
http://www.aircraft-world.com/shopdi...asp?search=yes 