They're nowhere near as loud as the real thing. You can fire them indoors without fear of bursting eardrums or scaring the neighbours!
Take a look at the Modelgun Video section here. Video cameras tend to muffle volume a little though so bear in mind they're a bit louder than this.
Prices vary tremendously but Japan and Hong Kong are probably your cheapest bet buying new.
Francky and shazhib (both members here) are really good and worth trying first to be honest.
There's
http://www.gunsnguys.com/home_ie5_1024.htm (check under Model Guns)
Also
http://www.wargameclub.com/ check Special Items have the odd bargain.
Cerwyn
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Hobby collector of mainly WW2 era uniforms, headgear, field equipment and replica weapons.