If you see a bunch of folks charging at you with knives either 1. Grab the nearest blunt objects you can find and CHARGE! or 2. Run away as fast as you can.
Try one of these. It will disappear in a flash in all but your small inside coin pocket. It is really much, much smaller than the picture and weighs only 11 oz, loaded.
It is guaranteed to take the hot air out of jihadist Muzzies.
Just make sure you rinse it well if you go swimming in salt water.