Also things like laying open the breech and setting off a White Phosphorous grenade there would screw it up pretty good.
I don't think white phophusurus burns hot enough to score metal and regular frag grenades won't work.
Magnesium/ thermite grendes are about the only permernant way to destroy a piece of steel like a cannon. Recoil mechanisms and carriges and firing pins can be replace.
For what its worth a US Tank of today has 5 thermite grenades as part of its combat load. If you need to abandon a tank to the enemy, you part one on top of each fuel cell in the engine compartment, one on the radios inside the tank, and one on top of the breech of the main gun.(4) total.
The last grenade I believe is to be used on any classified or secret material/equipment or maybe in case of failure of one of the other grenades. This info can be found in one of the operators/technical manual for most US equipment
Dig around I am sure the policies for destroying cannons in the Canadian army are published somewhere and probably haven't changed since the war. [/quote]