Seeing as this pistol is basically a .410 shotgun revolver AKA "The Judge". There are a few routes you can go it can fire shogtun rounds either buckshot or slugs. Pellets AKA BBs are used for hunting birds so they have no place here. Or it can fire .45 pistol rounds.

If you go the shotgun route the buckshot would make this revolver very similar to the shortstop shotgun for the scout. As far as damage and range capabilites, since I do believe the shortstop uses .410 rounds. Slugs are pretty much the same as a .45 round for range and power.

This could be the first shotgun weapon for the spy. I'd probably give it damage stats similar to the shortstop as seen below.
Base damage:48
Crit damage:144
[12 damage × 4 pellets] (this might as well be buckshot)

Here is my take on the stats

Buckshot stats

Has 42% faster firing speed
Deals 100% more damage per pellet
+30% cloak regeneration while deployed
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Has 60% fewer pellets per shot.
Has a 33% smaller clip size.
Cannot be equipped while disguised

Slug Stats

Crits on accurate headshot.
+30% cloak regeneration while deployed
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Accuracy decreases after initial shot. Has a short
cooldown during which it never crits.
–15% damage penalty
No random critical hits
Cannot be equipped while disguised