Seems difficult to do with out eliminating something like the 26. Just thinking out loud here but what if criteria was created for size, capacity, and weight? Create a box the size of a 26. Restrict magazine capacity of automatics to 10 rounds or less and revolvers to 6 rounds or less. Restrict unloaded weight to around 23 oz or less. The rest of the rules would match up with SSP and SSR.
I think the key is restricting the weight and size of the firearm used.