I am totally obsessed with cards as RPG aids. Ask me sometime on twitter why cards are better than dice and watch me go insane as 140 characters keeps me from explaining it fully. Also, every gamer loves a bagful of guns. These Arsenal Cards provide a solid aid for a historical game and as a result I'm giving it high marks.
The supplement begins with a glossary of what various firearm terms of the period mean, from wheellock to lever-action. Then a quick page or two of setup and you're into the cards.
The cards are meant to be printed on business cards - each is a color photograph of the appropriate gun on the front, and a description on the back that includes manufacturer, weight, trigger style, and ratings in Power, Recoil and Concealability.
One particularly interesting statistic is "Max PBR", which stands for Maximum Point Blank Range. Since in most RPGs you're dealing with close-range shootouts, the creator of the Arsenal Cards actually went to a gun website and calculated the approximate range that a shooter would consider "point blank" using each of these historical weapons' weight in normal shooting conditions. This is pretty awesome.
It closes with a table that will help a player or GM see and compare different traits of different guns.
Because it's not system-specific, players will still have to adapt these ratings to the system that they're using. That's fine - this is a very in-world description of the guns rather than pursuing specific stats. (Many of these guns would almost certainly have identical stats in some broad systems.)
Weirdly, there's a suggested table of penalties and bonuses when discussing ranges, power, and concealability, but no clue as to whether, say, a -1 penalty is on a 1d20 roll, on 2d6, on a pool of d10s or d6s being added or counter, on Fate dice, or what. This section is of no use and should be discarded, or it should be more fully integrated into a specific system so that players can understand what they refer to. If I had to name another way to improve the arsenal cards, it would be to include traced or more constrasting pictures, as the details of some of the pictures will be difficult to get in normal printers. Also I would have bookmarks for the guns and the glossary instead of the glossary and the card layout section, since most of the card layout is on one page.
Still, If you're not running a historical game, this supplement will make you want to run one with a ton of shootouts so you can scatter these cool pictures all over the table. Having one clipped to your character sheet will help you visualize the weapon your character's carrying in the terms they might experience - is it heavy? Can I conceal it? What exactly do people see when I pull it? Highly recommended.