I use a j-frame .357 with enclosed hammer, on an ankle rig. Reason: it is very very powerful, utterly simple, and relatively small.
Somethings to consider, however (some of which have already been mentioned):
1. What regs does your department have on backups? If you have not yet been hired on, don't buy your backup yet (sounds silly, but I know guys who did it, and bought a gun they couldn't use).
2. I agree with what was said above about like-functioning guns. If you get a semi-auto, get one that functions _exactly_ like your duty weapon, so in a high-stress situation you don't start doing silly things like fumbling with safeties and mag releases and take down levers and whatnot. Interchangable mags and like-caliber is a bonus, too.
3. if you do get an ankle rig, make sure that 1. the gun sits in it very tight and 2. the rig is very secure on your ankle. Why? As a cop you will occasionally have to chase people (no! Really!), and if that puppy ain't on your ankle VERY FIRMLY, you will be walking back over your path looking for it. Thats something you don't want to have to explain to your sergeant. Galco makes a decent ankle rig with a tensioning adjustment , and I have yet to lose my backup.
Mike
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"A human being should be able to change a diaper, plan an invasion, butcher a hog, conn a ship, design a building, write a sonnet, balance accounts, build a wall, set a bone, comfort the dying, take orders, give orders, cooperate, act alone, solve equations, analyze a new problem, pitch manure, program a computer, cook a tasty meal, fight efficiently, die gallantly. Specialization is for insects." -Robert Heinlein