M3 Grease Gun
Looking for something different to carry? This might be the ticket. This version of the M3 is a gas blowback system. Gas is stored in the magazine and the bolt actually moves with each shot providing a very realistic shooting experience. Though it's entirely of plastic, you can't tell until you pick it up. Probably not to most rugged weapon, but the best part about this weapon is its price, the version you see above can be had for about $150. A "limited edition" sillenced version will run you about $100 more.
Where to Buy
This one isn't too common in the US. Check the dealers from Japan or Hong Kong. Your price will be about $138 before shipping and Customs, $206 for the silenced version.
Other Information Sources
Some helpful sites and reviews of this weapon:
Flange's Arsenal Review