Quickdraw holster does a nice thing:
It saves you a fight action.
It means that your first gunshot per turn is a free action, use it with a Lupara or Sawed-Off Shotgun, maybe Vicious Blow, to nuke moderate enemies or chunk a boss right as you step into their location. Use it with smaller guns like .45 Automatic to do 4 fights in a turn or clean up chaff efficiently.
But.
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It costs 4 resources.
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It does nothing by itself. Nothing! You still need to find the gun and pay for it.
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It's 4 XP.
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It has trouble with timing. (if you have, say, some tools or a 2-hand gun in play, playing a 1-hand gun will kick out something existing.
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Again, it does nothing! Often support cards that supplement a playstyle come with some niceties like a health or sanity tank, bonuses on tests involved.
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It costs 4 resources, again.
All that put together, Quickdraw Holster is very much the cumbersome capstone to the whole "1-hand firearm" archetype. At which point you might ask, "what, there's a 1-hand firarm archetype!?", which is a good question, because there is not!
Maybe if there were better 1-hand guns around with useful and intuitive support this card would be useful unto itself, a capstone to a build, but in the current cardpool this is a lone wolf with no pack to hunt in, But until then, you'dd be better served spending these 4 XP on Charisma + Leo De Luca, they wont get you that extra hand slot, but it'lll get you that extra roundly fight action, and then some!