So the shells for movecraft are... a bit odd sometimes. Take the heavy tank shell. It uses three steel ingots as the shell itself and... three iron ingots as the casing. Using iron instead of brass is a bit odd, but the shell is powerful so it isn't totally unrealistic. Then we get to the normal tank shell. It uses SIX iron ingots as the CASING, and then a... BRASS ingot for the shell? Now that makes absolutely no sense, using a softer metal as the shell instead of the casing. Not only does this mean the tank shell unrealistically crafted, it is also unnecessarily expensive for its firepower. One more example. The simple "shell" ammunition is ridiculously expensive in iron to produce compared to things such as tank shells or 4 inch shells for the same, if not less, firepower. Again, we see that the casing for the shell is made of iron which makes no sense at all.
I've made suggestions before relating to air-launched missiles requiring circuit boards even though /lockon was disabled in nations and how that was dumb. I just think that there are some simple changes to shell crafting recipes that are needed. It wouldn't really affect balancing at all because you still need a decent slimefun setup to create shells if my suggestion is added.