"Serious Contender" types -- the recoil from this one STINGS:
Hopkins & Allen .32 rimfire, 5 shots
It's more accurate than the S&W Model 36 (prohib) above, shown for size ... Antique, not even a PAL required (though you need one to buy ammo).
There are plenty of these out there, and of course smaller, more exotic revolvers in the "velodog" format
Here on a 1" grid are an antique Belgian .380 revolver
compared to a more modern FN Belgian .380 (prohib).
The FN can be converted to restricted by putting in a 1" longer barrel.
The .380 revolver needs nothing to be unrestricted...
In order of cost, the smallest legal "modern" revolvers/pistols will be the North American Arms .22 / .22 magnums such as Kodiak sells.
In .380, the FN 1910 with a 4.25" barrel (converted to restricted) can be had for about the same price (much more expensive to shoot though).
Then a SIG 239, and the HK P7 PSP (M8 is slightly larger) in 9mm for top dollar.
The biggest bang for your buck, literally, would be one of these:
Colt's .41 rimfire, selling for wildly varied prices,
will cost you about $3 a shot but the tiny under 2" barrel
sounds more like a canon than anything I've fired yet.