Originally Posted by Zeroroaster
Li-po's will deliver too much juice for your gun's guts. They're also a bit of nuisance to handle and store. You can't just leave them lying around. They need attention and do pose a burst risk. If you don't need a Li-po, don't bother, really.
I run just ordinary 9.6V Ni-Mh. They have little or no memory, don't care how they're stored, readily available and cheap. If you need more performance, optimize your air system and leave the juice alone. I have a G36 shooting 428fps with a 9.6V Ni-MH and it only has a 110 spring! Make it more efficient...otherwise it's no better than running a big block in a Toyota. No point in having all that power if you can't put it to the ground.
Current lipos (quality products), are not unstable and are certainly the way to go. Lipos have definately come a long way, but they are light, well balanced, don't get memory, small and work well in colder weather than any others. It's imperative however that you have a good balance charger to go with them.
Hobbyking carries Turnigy-Nanotech lipos for under $10 US a piece and they are extremely solid. Just make sure to order from on the US warehouse option, or else you may be waiting awhile for your order. Also, they carry the IMAX B6 balance charger (as stated above), it's also very reliable and does a great balance charge.