Any advice for how one would go about figuring out what stats a weapon ought to have if it isn't one the rules already cover? As a thought experiment I decided to try building some characters from the Berserk manga, and while most of them are pretty easy since they use standard armaments and I can use the effect-building systems for the supernatural stuff, Guts is an outlier.
First of all is sizing up a pre-existing weapon - he uses a great sword called the Dragonslayer which nobody but him is strong enough to use effectively because it weighs 400 pounds, since (per its namesake) it's designed to have enough swinging force to cut through the natural armor of dragons, demons, and other supernaturally resilient enemies. So I'd increase the Minimum Strength column probably all the way up to +6, as I'd probably build Guts with Giant-Blooded and Great Strength x2 to cap him out, but what should that do for the rest of its stats? I'm not sure whether the greater reach of the weapon would increase its Initiative or if the huge inertia would lower it or whether those effects would cancel out. Same with the Attack & Defense modifiers, really - the blade is so wide that Guts effectively uses it like a shield on occasion (using the flat side to deflect arrows and such) and it gives him a reach advantage so maybe it's a plus, but then again it's harder to use with the dexterity of a normal blade. The only sure thing is Damage going up, but even then it's hard to decide exactly how much is appropriate. This is probably the most YMMV element, but if anybody has ideas for a ratio of how increasing the size and weight of a weapon would affect its stats, I'd be interested in hearing them.
The second one is a whole thing in its own right. Some of Guts' arsenal are things the rules cover, throwing knives and a crossbow built into his prosthetic arm, but the big one is the literal hand cannon (not like "predecessor to modern guns" but like "his hand detaches to reveal that the forearm is a cannon"). Cannons in general are a little post-Ars, never mind one built into a metal prosthetic. Normally I'd just give up and design the arm as an enchanted item, but the problem is that Guts' whole concept is based around using mortal ingenuity and iron willpower to fight supernatural beings; not only is the arm not magical in the source material, but more importantly Ars Magica's way of handling Magic Resistance basically means he can't be dependent on anything vulnerable to that. Arguably could use the "Finesse to aim" workaround for that, maybe? In general Ars Magica keeps the weapon system pretty minimal and leaves all the fun effects to the supernatural side, or else I could also have fun with the fact that enemies who aren't killed by the cannon tend to be blinded/staggered if it's used at point blank (either by the explosion itself or the small cannonball). Or I could be being silly, and maybe one of the supplements already has something I could translate into this? Fishing for ideas here too.