The thing is most people get it the wrong way round. The ammunition is designed first, then a gun to chuck it out of. The need is to defeat the oppositions latest armour. So a Ballastician designs a new shell and propellant, the enginner then has to design a breech to hold the charge in,physical and explosivly, a new rifiling set, a new barrel to stand the weight and speed of the projectile, consider that you are chuking a lump of metal from standstill to 1000g in les than a heart beat. Then produce design drawings, then manufacture one and fire the beast, then sort out all the little problems you hadn't considered. Then re tool then allocate materials, including somewhere a new alloy or process, bet on it. Then start changing the vehicle to fit the whole new contraption. That it happned at all in five years is a miracle!