I'm bringing up this old thread because I experienced trouble with this, too. The core problem is that FRED seems to calculate in units of 1000 when you manually change the bank's capacity. But one thing at a time.
What I have is this:
$Subsystem: turret01-torp, 0.8, 3.0
$Default SBanks: ( "Shark#Three" )
$SBank Capacity: ( 100000 )
$Flags: ( "turret use ammo" )
Initially, Shark#Three was a weapon that fires voleys of three missiles where each missile has a cargo size of 10.
Now if I put a ship with that turret in a little test mission and open the weapons tab it shows me a capacity of 10,000. Because FRED always shows the actual number of missiles you can fire and not the capacity and 100,000/10 = 10,000.
In a specific mission, I want that turret to fire only ten volleys which is a capacity of 30. This, however, is always rounded down to 0. If I enter values >50, they are rounded up to 100. If you then enter the mission in the text editor, you see this:
+Sbank Ammo: ( 1 )
So I went on from there and put a 3 in there. This gave me a capacity of 300 missiles. So +Sbank Ammo: ( 3 ) seems to be qual to $SBank Capacity: ( 3000 ).
I then went further on to change cargo size to 100 in the wep.tbm and opened the mission file in fred again. It then showed me the correct number of 30 missiles.
As for the situation Black Wolf described over four years ago, the FusionMortar's cargo size also needs to be so that you get 75 as a division out of a multiple of 1,000 and the cargo size, e.g. 40. (3,000/40 = 75)
I don't know if what I say is fully correct. It's what I deducted from me playing around with it. Anyway, changing the turret ammunition in FRED by the looks of it gives you a totally different calculation formula for the actual number of missiles. And I don't think that this is a situation we want to maintain.
I really hope that one of the coders can say something about this.
Supplemental: The turret also seems to be able to fire one more volley once capacity is down to 0. So it always goes down to -3.