I tend to change batteries at lunchtime no matter how many shots fired so just buy a couple of batteries that fit and swap them. I tend to go on the 1 shot per mAh formula which allows me to easily get away with 2 x 1400mAh batteries
If you go for buffer tube batteries, don't get Nuprol. They're advertised as 1400mah but only actually manage about 1000mah. The Turnigy stick lipos are a lot better.
Turnigy 2000 mah crane stock 7.4 v...cost 8 quid with deans connector from hobby king. Fit easily in the stock of the krytac. I have a vp 2000mah crane stock battery that cost me £17!!! Which is about the going rate for Airsoft shops. Obviously it'll last a few hours but if your hammering full auto it won't be as little long.