[Legbreaker]

Of course the vast majority of actual artillery rounds needs to spin to arm, so it wouldn't matter if it didn't clear the barrel.

I went through a similar thought process a couple of months ago for a game while trying to come up for an idea to deliver gas bombs onto a target. A grenade with pin removed, stuffed into a foam sabot (taking great care not to release the pin) then shoved into the barrel should do the job nicely. The same arming system could be used for larger charges too with a little creativity (not exactly safe, but could be very effective).

Then there's the old staple of shoving grenades into glass jars (ala Airlords of the Ozarks). They could be flung by catapult, staff sling, or any number of other methods. Doesn't even need to be a grenade (take the wojo incendiary grenade for example - two chemicals which mix when the jar breaks with an exothermic reaction).

So, in answer to the original question, damage is purely dependant on the charge or projectile. The only question is over the delivery system (range, accuracy, power, manoeuvrability, etc).