The way I'm going to review this card is by re-interpreting the text into three separate texts, one for each of the outcomes from success (every weapon does the same thing when you fail, so that's an un-necessary consideration). I'm also going to interpret "succeed by X", as "this test gets +X difficulty". This will turn a review of one weapon into a review of three weapon with direct comparisons to other existing weaponry, in an attempt to gauge the power-level of this card:
First, say you have a heavy aversion to risk, and really just want the cheap +2. This weapon might as well read "Spend 1 ammo: FIGHT, You get +2 for this attack", in this case. There exists one other weapon (as of Dream-Eaters) that provides a +2 boost without additional damage: Gravedigger's Shovel. A quick comparison shows that the imaginary .41 Derringer falls way short: for one less resource and two less exp, you get the same boost an infinite number of attacks, plus a little extra versatility to boot. So clearly you won't be getting this weapon for it's base effect.
Next, let's say you really like the un-upgraded .41 Derringer, and care little for extra bells and whistles. In this case, the text is equivalent to "Spend 1 ammo: FIGHT. You get +1. This attack deals +1 damage". Sound Familiar? It's a .45 Automatic, albeit for for less resource, and one less ammo. Sounds fair, right? Except the .45 Automatic costs no exp, so to be equivalent in power to a card that costs 2 less exp is a bad sign, and thus, this imaginary weapon also falls short.
Finally, the big one. "Spend 1 ammo: FIGHT. You get -1. This attack deals +1 damage. Gain an additional action (limit once per turn)". This would be the only weapon in existence that would reduce your for the attack, if it existed. And yet, if you're gunning for the biggest prize, and as proven in the prior example, you'll be playing with an inferior weapon if you don't, then you have to consider the difficulty value as three higher that it's actual value.
If, say, you want to succeed despite drawing a -3 (the staple standard for tests at medium/hard difficulty), then you want to have a final at 4 HIGHER THAN THE TEST. Not impossible, but damn are you shooting for the moon for this extra effect.
The funny things about rouges is, despite having the second largest select of weapons to guardians, not one of them have a value higher than three (until the Dream Eaters came out, I'm getting to that). Of the rogues that do have 3 , "Skids" O'Toole has access to guardian cards, Jenny Barnes can pull off some magic with Money Talks and Well Connected, and Finn Edwards does like his illicit card synergy. Whether that's enough, I'll leave to you, because personally, I'll not sold on the idea. A better candidate would be Leo Anderson, especially with Beat Cop, who, with all the expensive allies he'll undoubtedly have, might appreciate a cheaper weapon alternative.
...And then along comes Tony Morgan, blowing all other candidates out of the water. Unlike the others, he's a combat thoroughbred, unlikely to be doing anything other than fighting (don't you dare mention seeker Tony). He alone can make .41 Derringer work, in my opinion, and not a moment too soon, as I like the "succeed by X" archetype for rogue, but never really got to see the fighting version of it (alongside Switchblade), until now.