I know its old but http://www.smh.com.au/ffximage/2006/..._gun_sight.jpg

Too bad you can't aim worth shit like that unless the target is less than 5 feet away, which in airsoft, if somebody is that close behind you, you are already dead and/or they are within the MED and you aren't allowed to shoot em anyways.

You could. You won't be able to get a continuous stream of fire on your target though, as your gas will basically run dry after a fraction of your mag. Sure you could practice it, but you will likely never use it, and may damage your internals while you're at it? I don't get the point of it, in that sense, lol.

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.

The urban prone will require you to shoot your weapon on its side.....and that particular position is somewhat useful with barricade...

FYI, only guns running WA's Magna(maybe other version as well, but Magna for sure) will allow for non-vertical mag without the gun just dumps propane out. Mainly due to how their mags are designed. As far as GBB goes pointing straight(or generally passing 30 degree from horizontal in some cases) up and down will likely have the same issue also with liquid propane dumping.

My friend shot his kjw g23 with a plastic slide upside down... the slide flew off of the pistol and damaged a few parts.