Allow me to present a case for the .45.
While it is certainly, not a two handed boss killer weapon, it does have a few very specific builds that will reach for it. Most of them are out of the "Guardian" faction.
So Mark will probably go for something else.
So when is this card worth it? It probably is worth the slot if you can get multiple, efficient uses out of it OR if you really cannot manage something better.
In guardian a Sleight of Hand build out of Zoey might find slots for this card. Being able to pop a card out for those turns were you are swarmed that packs a pretty decent punch is probably worthwhile. It is also a pretty great sidearm to a two-handed weapon and Zoey can pay for the gun out of hand with her additional resource generation.
Lola also really appreciates this card and the newly spoiled level 2 Derringer. With Sleight of Hand Lola can potentially play out a selection of weapons that will allow her to "punch up" to a five fight value and reliably do +1 damage. With her 3 base fight, this is pretty huge. This combo also requires minimal role switching (as Sleight of Hand allows you to "cheat an item out and then you can role switch to that item).
Filling the role of a secondary monster hunter, Skids might also appreciate having a second, third (or fourth with the typewriter) target for his sleight of hand. "Cheating" cards like this and the derringer would allow for some fun gains in resources when combined with "Watch This!".
Finally this card is quite a decent pick for Yorrick. For one more cost than a Colt you can ignore retaliate, do an extra damage and do +1 damage. It would not be a mainstay weapon in his builds, but a firearm is always appreciated.
To conclude my case, I am going to make it clear that I am not advocating most Guardians choose this weapon OVER a shotgun or a lightning gun. Those cards are FAR, FAR more efficient. However if you are a character that can play sleight of hand and/or you are playing Extra Ammunition anyway, this card (and its sibling) might be worth a look.