The issue with the Preamunitus skill

One thing I’ve noticed about some skills that improve magazine size, specifically Zane’s Praemunitus is that they seem to underperform, especially on the kind of guns that you would like to boost the most. Praemunitus grants a boost of 8.3% per rank up to a max value of 25%. This means, having a single point in Praemunitus won’t benefit you at all with any gun that has a mag size smaller than 13. That’s pretty much all Jakobs and Torgue pistols in the game, the majority of snipers, the majority of launchers, a considerable amount of shotguns. Praemunitus is only really worth it if you spend the full three points, ideally you wanna boost it by a COM, too. But the thing is, that’s just not worth three skillpoints. If it were a single tier skill that increased your mag size by 25%, maybe that would suffice. But even that might not be enough.
I don’t think there’s many people out there using this skill, not only because you can get a much higher mag size bonus from just an artifact which still leaves space for two other bonus rolls as well as a primary and secondary effect - Praemunitus also competes with Synchronicity and Borrowed Time, one being among Zane’s most important damage buffs and the other one is at least a somewhat popular choice for people who want to get more out of their good misfortune.

What I would suggest is the following:
1: Make every point of Praemunitus increase your magazine size by one, if the mag isn’t big enough for the percentage value to have an impact. I don’t think three extra shots in the mag will be gamebreaking, but at least they’ll make a bit of a difference if I’m using something like a Duc or a Maggie.
2: Increase the percentage value. I’d say at least 10% per point, maybe even 13.3% at which point the full bonus would be about the same as you can get from a level 50 artifact.

I still don’t think that would make Praemunitus a top tier skill for Zane, but it would be one worth spending points on.

Hm, I think the only thing that needs to be fixed is the full 25% instead of 24,99%.
Adding up to 3 shots would be very strong, but then useless for high mag weapons.
Having 100% increase with relic and skill would probably be too much.
I think it’s not coincidence that it is not possible to double your magazine that easily.

I mean the 3 shots as a minimum. So the first rank would (if we stick with the base values) increase your mag size by 8.3% or at least one shot. So with a really low mag weapon like the Duc or the Hellwalker, you’d get one more shot out of it whereas with a really high mag weapon like the Shreddifier or the Magnificent you would get the 8.3%.
The one bullet per rank minimum is just because, as already noted, the skill otherwise simply doesn’t affect a lot of weapons at all if you don’t go with the full 3 points and even then, the bonus you get isn’t worth 3 skillpoints.
“Too much” seems very relative to me. I mean, Zane benefits from larger magazines a lot less than for instance Moze who can essentially regen ammo at a faster rate than she consumes it when her magazine is large enough. Zane simply gets some more shooting time out of it.

Some weapons are just meant to have on shot and stay that way. Being percentage based is probably very intentional. Sure having more shots one punch chump and flakker and what not is cool. But balancing around every weapon being able to have +3 shots would be very tedious I guess.

Maybe, but 25% mag size for 3 points is still complete and utter ■■■■■ which is the reason you hardly ever see Praemunitus in any of the more popular Zane builds, even though mag size is such a big deal in the meta of this game.
I also wanna point you to Moze again. They had no issues balancing around every weapon having reduced ammo usage, permanent ammo regeneration, the ability to temporarily not use any ammo and considerably increased mag size, depending on the exact loadout.
And Moze is still reasonably balanced and by far not the most busted character in the game.
Do you really think that three extra shots are gonna make Zane entirely broken? Mind you, Zane has a pretty decent reload boost with cool hand, to the point where reloading inbetween Flakker shots doesn’t really reduce his effective rate of fire because the Flakker, even if it had a much higher mag size, is still a very slow gun. That goes for the majority of one shot guns, they do have a very low rate of fire so they aren’t overpowered if their mag size gets boosted. The Flakkers mag size got nerfed specifically because Moze could boost not only the mag size but also the damage and the rate of fire by a lot, other characters were hardly affected by it.
Fact of the matter is, as long as Praemunitus is the way it is, it might as well not exist. Any of the guns that have such big magazines that they actually get a noteworthy boost out of Praemunitus are not ammo efficient on anyone who isn’t Moze and they are not the kind of gun that would be good on Zane, anyways. If the skill isn’t actually relevant for his preferred weapon types there’s no point in having it.