Because the stock is adjustable for length with the use of stacked plates, a battery doesn't fit there. you're still stuck with using a mini in the handguard. You could probably modify it to fit, but since it's a sniper rifle, you really don't need a ton of run time.
If you want to use a large battery, go with the G&P Norwegian type stock. I've got a couple of them with 9.6v batteries and I usually charge them every other game of so.
