Disclaimer: Pre-Scarlet Keys release review.
This is quite a niche card. For it to work, you need a Firearm, of course, but also to be spending your latest Ammo, while still having at least one fight action to take.
That being said, I believe that we can all agree on the fact that Vicious Blow is a must-have lvl 0 card for any main fighter Deck that has access to it. So if you pack at least 4 Firearms in your Deck, meaning you will use One in the Chamber at some point, how do these 2 compare?
Vicious Blow is simple: +1 , +1 damage on success, since you add it to another Attack.
One in the Chamber is fast, adds 1 Ammo, to your gun, which means at least +1 damage to an attack that would have been performed with bare hands, and +3 .
Results: Vicious Blow saves you one action only with enemies that have odd health, when One in the Chamber helps better (more ) with 4-health enemies and bigger, or if you are playing with 3-4 investigators and there are higher chances that you have to deal with more than 1 foe at the same time.
Compared to Extra Ammunition, you obviously get 2 less Ammo, but it is fast (meaning no AoO), and gives you a significant boost for the next attack, provided that you attack the same round. I see One in the Chamber like a great placeholder for Extra Ammunition for at least 2 thirds of the campaign.
The Firearms that are happy to have One in the Chamber are:
The happy users are:
Leo Anderson/Tony Morgan/"Skids" O'Toole that have access to the "Succeed by X" guns or tend to also purchase massive Firearms. After that, any that is tempted to run the Shotgun like Sister Mary, Mark Harrigan, Holy Shotgun Tommy
Conclusion: Fair card, will see use in certain Decks that plan on using the Shotgun, or the set of Succeed by X Firearms. This will most likely be a placeholder for Extra Ammunition, which makes the upgrade a lesser priority.