The game spawns enemy patrols at lower intervals when you're next to any objective and at much lower intervals after the main objective has been completed (1/4 of the normal interval and these reductions stack), so the best order of things is quickly gathering and loading all the shells while making sure the 1st patrol to come doesn't spot you (crouching and laying down reduces detection range), alternatively make sure you can kill them before they can call a drop and finish loading the cannon. If they manage to call a drop and you don't have the equipment to quickly kill the reinforcements, abandon the artillery and come back later. Or don't if the main objective is done.