I've shot a springer pistol sideways for fun and I don't recommend it. For one this the hop up on most springers are fixed (ie. generally tuned for a specific weight for most conditions (I believe it's .25)) so the shots always curve to the side.
On a GBB I highly don't recommend it. For one thing in the mag is liquid propane and you want to turn it into a GAS to be fed up and expand and the only way gas can do that is if the liquid propane is on the bottom of the mag (gas rises so upsidedown liquid propane will be at the "top" of the mag and just spew out from the "top" of the gun). Couple that with the "cooldown effect" from a liquid turning into a gas like that and you have a good chance of freezing/breaking your internals.
ಠ_ಠLess QQ more Pew Pew
READY TO >> RACE