This will absolutely fit inside of the buffer tube:
http://www.khmountain.com/airsoft/pr...oducts_id=1171
It's a batter that can only provide 16.5A thought, and it's not even confirmed if it's continuous or peak discharge rate. Sadly it's one of the few lipos guaranteed to fit inside.
The next one is likely to fit inside:
http://www.khmountain.com/airsoft/pr...oducts_id=1175
If you have a charger for it, I would of course recommend that you go straight for lifepo:
http://www.khmountain.com/airsoft/pr...oducts_id=1184
Here are all the lipos/lifepos on that particular store:
http://www.khmountain.com/airsoft/ad...i-poly&x=0&y=0
Sadly I 'm not happy with the store though. Ships awfully late, slow and horrible communication. Still has some of the cheapest batteries I've seen yet.