I've had two artillery pieces, one size 4 and a smaller one, size 2.5.
When using both, only the small one fires. When using just the size 4 one, it won't fire at all. I suppose I can use only up to size 2.5 or something, though it looks like a bug
EDIT: Found another bug: when using cooling subsystem on artillery it refuses to fire, so probably the bug is when the program didn't espect me to use two artillery pieces and cool them both, that's why the second one works I guess
EDIT 2: That's not it, this time I put in 3 artillery pieces in the station, all cooled, and all fired. The bug is erratic and seems artillery gun size seem to matter and a combination of using cooling system on it while there's another artillery weapon in the design. Don't know...
EDIT 3: I noticed when I put 3 artillery pieces in one station, the third if it has a cooling subsystem, it doesn't register
EDIT 4: found the bug rules, if the cooler makes the refire rate less than half the original, the artillery won't fire (also if it's size 4 by itself it won't fire, size 3.75 works)