I have three different frequencies in my inventory, but, I can order crystals for three more from HK for a total of six different frequencies. (It takes a week and costs $10).
I would like to officially discourage the use of these with any sort of real metallic ammo
Some folks have used 6mm paintballs for battles with these. I don't really warranty the use of those because the quality is so inconsistent and a burst one can gum up the works.
If you are interested in something larger and more robust you should look at the Tamiya 1/16 scale tanks like these....http://www.tamiya.com/english/products/56010tiger/tiger.htm They have a pretty neat battle system that registers hits, causes the hit vehicle to stutter or stop, etc.
There's a lot of hobbyists that build and fight these things on 1/16 scale battlefields like the one at the AAF museum up in Virginia.
At about $1,000 + labor to build them they are a little rich for my blood.
Come on back when you get some bucks!