You probably need a flat headed projectile to produce that level of damage.

@Background Pony #2FC6  
LOL WHOOPS. Facepalm I did this quite a while ago so I don’t know wtf was going in my head

Notice that in the numerator you put 2900sqrt(335)*; since the first number is velocity in m/s, shouldn’t it be 823 instead of 2900?

@koznyshev  
That seems to be the case. The closest real life equivalent to Ponk’s cannon would be a Carronade.  
The relatively lower muzzle velocity and heavier shell weight of Carronades compared to long guns of the same weight class would explain the lack of overpenetration (the actual shell itself itself is nowhere to be seen outside) and amount of destruction the cannon could do against non-steel soft targets as exhibited in the comics.
 
https://en.wikipedia.org/wiki/Carronade

Pretty sure that thing’s actual power IRL would be more equivalent to a blackpowder cannon, especially since it’s muzzle loaded so you couldn’t even get your supposed AP shell in there. It’s just not a cannon that can take cartridges like that. I think the issues with this are made at what ammunition it’s firing, and it’s cartoonish aftermath. Also, cast iron cannonballs don’t need to explode, so it wouldn’t be wrong to assume it’s just cast iron.

ITT we overanalyze a two-panel throwaway gag until it ceases to be funny.

Since when did Pinkie Pie or her stuff make sense?  
However, I like some fandom geekery sometimes too. Well done.