Why even bother playing if you just want to dupe an OP item then beat the final boss ridiculously fast
Again, the quantity of available ammo isn't really the point. It's the game value of the ammo. Terraria doesn't normally assign money value to damage so directly (except with the coin gun, but I'll get to that), but just so we have some numbers to work with, let's pretend they did.
If Bullets Were Bills
Let's say that if each musket ball does 7 damage and the Arms Dealer sells them for 7 copper, then each point of damage is conveniently worth 1 copper. So now let's look at this gun that does a percentage of the enemy's health, rather than a discreet amount of damage. Again, for easy rounding, let's say the bullets do 5% of the enemy's health.
Against the Desert Scourge, which has roughly 4,000 health, the gun does 200 damage. In this theoretical example, then, each bullet, which we'll call OPA (Overpowered Ammo) from here on, is worth approximately 2 silver.
But then let's look at Yharon, who has 4 million health. Still doing 5% damage, the OPA will deal 200,000 HP worth of damage. In this case, our ammo is worth a theoretical 20 gold each.
So we already see a massive discrepancy in values between fighting an early boss versus fighting a late boss. If the ammo is priced to be viable against an early boss, then it's far, far too cost-effective against later bosses. If it's priced to be viable against late game bosses, then it's worthless against anything before that boss due to cost inefficiency.
The Coin Gun Case
Now of course there is a weapon in that sets a precedent for money equaling damage. The coin gun, which is one of the highest DPS weapons in the game in the vanilla game, decided long ago that one platinum is worth 200 damage! So even within the niche of money-powered weapons, there's a competitor that was already balanced for vanilla and it does a discreet amount of damage. No matter what you're shooting at, whether a slime or the Moonlord, the Coin Gun does 200 damage per platinum.
So supposing we could obtain the Coin Gun at the same time as we can obtain OPA, it's very easy to examine the competitive values of the guns. Let's look at the Magnum OPA, which we'll round to 5% damage again for easier math.
Again, it does 200 damage to the Desert Scourge, but each bullet costs 6 platinum. Compared to the Coin Gun which does the same damage for 1 platinum, the OPA is 1/6 as efficient at the job and is therefore totally unsuitable.
Against Yharon, the OPA is doing 200,000 damage per shot, or approximately 33,333 damage per platinum. Compared to the Coin Gun, OPA is 167 times more cost effective in spite of being a pre-hardmode weapon. So where does OPA break even and fall in line with vanilla balance?
Well, it never quite does fall into parity with the Coin Gun, but the closest point is when fighting Cryogen, at which point OPA is doing 1,680 damage (or 280 damage per platinum) against Cryogen's 42,000 total HP. After this it begins to tear away from the Coin Gun, becoming more cost effective as the game goes on.
The Coin Gun can be acquired at the start of hard mode, so the only thing saving it from immediate obsolesce is its DPS now. Its damage per platinum is inefficient against OPA from the second you an acquire the Coin Gun. So again, because I like round numbers, if you compare the Magnum to the Coin Gun, the Coin Gun fires three times faster. So at what point does the DPS of OPA exceed the Coin Gun?
Well, it varies a lot because bosses start appearing in pieces or aren't "organic" after Cryogen, but by Duke Fishron OPA is both substantially more cost-effective than the Coin Gun and it's also doing more DPS. Granted there are upgrades to the magnum for some reason, so in reality an upgraded OPA likely overcomes the Coin Gun a little sooner, or at the least simply flattens Duke Fishron all the better.
So, Balance?
If all the above should illustrate anything, it's not how to price the bullets, nor is it about how to restrict access to the bullets. It's that percent damage will reach a competitive point at some specific section of the game, and then gradually grow more powerful from there on. You can't really balance that, because OPA will only be balanced at the specific break-even point where it's competitive with other weapons of the local tier. So if you wanted the gun to be balanced at around that discreet tier, why would you use percent damage at all? Why not just give the weapon damage that's competitive for the tier?
I personally suspect the only reason we're seeing percent damage emerging in the mod is because Fabsol is bored, is having a difficult time arranging new bosses, and is trying to shake things up with new weapons. But he can't merely add normal weapons, because then they'd only be competing with all the weapons that
already exist at those tiers, so instead he's thrown in this odd, floating weapon that kills 5% of a boss per shot.
Okay so after the update, mod randomly working now. It is actually kinda sad how happy I am right now.
It's not exactly random! The Planetoids update broke the game by expanding the size of the world gen, which caused out of memory exceptions for slower memory cards or for people running multiple mods. The latest update rolled that back, so until or unless things get more bloated you'll be able to play Calamity. I
strongly recommend keeping a backup of this mod version (or whatever version you like the most) in case a future update causes problems again.