Help on destroying the Bruno Railway gun.

[bringonmow]

I'm on the Bruno mission and have completed everything except destroying the gun. I've found the dynamite and placed it in the "inventory" of the gun. Is this the best way to destroy it? I've tried taking the dynamite and laying it near the gun, and I've even destroyed the guns' shells to use them as a chain reaction.

I'm probably missing something simple-but how exactly do you destroy the gun?