not optimal, but we are on track. a revision can make it work greatly.
I see that ammunition is a big issue, but we can improve it. the attending mage can also be a problem, but might not be too big.
As for ammunition, the problem seems to be making it fit the barrel well, not making it crack, perhaps making it lighter to achieve greater range.
great news, we can do that! crystal summoning is the perfect solution. Easy, cheap, can control the shape, sturdy but light.
Now, what it is that I hear you say? crystal disappears in their antimagic field?
-evil grin-
that is a feature. our new projectile can be formed by a crystal matrix, in which other objects are embedded such as smaller rocks, pebbles, fire venom from the wasps, nails and metal scraps depending on what is needed by the situation. if they don't dispel it, they get it by a big crystal ball which may shatter on impact.
if they DO dispel it, they get it by flying shrapnel/ sprayed by venom/ hit by biggish rocks depending on the shot we use.
Crystal cannon ammunition rocks are nice, but they don't quite cut it. we need something light, smooth and sturdy enough to not crack in the cannon. The solution adopted is creating a crystal sphere which encases the actual ammunition, which at this point does not need to be carved accurately. the ammunition encased can be a big rock for concentrated damage, several biggish rocks, pebbles or nails and metal scraps /small metal balls if we want to cover a big area. other special ammunition is considered, such as 2 chained balls or bottles of fire wasp venom.
In case the solid sphere is too complex, an hollow sphere is cast. smaller objects can be inserted after casting, while for bigger ones the sphere is still cast around them.