6 mm
I don't no about spring, but the Green Gas M700 Sniper Rifle Competition Professional Airsoft gun, has 720-FPS!!!! And the, Spring CSI L96 Bolt Action Airsoft Sniper Rifle, has 550-FPS. They are both from, Hobby Tron.com
the best way to lower the fps on a sniper rifle would be to reduce the type of spring you are using. airsplat.com has a whole bunch to pick from to decide what type of FPS you're looking for.
300fps- 400
The answer to your question is up to 275 fps. ( Feet per sec. ) For more info on this airsoft pistol see related links.
No "good" pistol like that exists under 50 dollars.
at minimum 330.
Yes it is. It has 410 FPS and It shoots 150 feet. Be careful at close range though. It can make people bleed.
Depends on what you will use it for. If you're using it for CQB skirmishes, 300-350 fps is good enough (of course with appropriate weapon i.e P90, MP5, MP7). If you're using it for middle range urban/woodland skirmishes (i.e. with M4/M16 family, G36, SCAR, HK416), you can go for 400-450 fps. If you're using it for sniping (Bar-10, M40), you can go up to 600 fps.
280 fps
The best airsoft spring pistol for young kids are guns that aren't extremely powerful, I'd suggest this gun which is at 235 fps, others will feel it but it won't hurt too bad. This gun may be purchased from Hobby Tron at hobbytron.com/AirsoftGuns.html, this is the title of the gun 'Sprind Tactical TMP Machine Pistol FPS-235 Electril Sight, Silencer, Flashlight Airsoft Gun' also, this guns runs around $15.95.
General rule of thumb is buy the best you can afford for what it is you want to do (target shooting, hunting, display, collect, etc..)