My pistol arrived on a piece of liverock (actually the live rock that the Zoos were on).
Here is some information from
http://www.blueboard.com/mantis/pest/tell.htm
"I have read somewheres that one way to tell from the clicking noises is that pistol shrimps tend to snap in ones and twos, whereas mantis shrimps make rapid multiple hits. "
"Pistol shrimps make those clicking noises by using one of their specially modified claws. Based from different sources, these are either used in mating, hunting (by stunning small prey), and in fighting with other pistol shrimps. Mantises cannot make clicking noises per se.... instead, any clicking noises from a mantis shrimp are due to using its forelimbs to hit something else, either inanimate objects or (more likely) hardshelled prey such as snails or hermit crabs."