I do have something to add as an idea.
What if you had a side panel on the lumberjack that consisted of gauges (that are consistent with how many shells each clip holds at once) and after you fire a projectile the gauge will light up and start moving a needle that counts down the seconds needed for the mortar to be armed, and then it will change color (like a red to a green). Since I know that there is an ability that increases clip (but am forgetting the name atm) you can have deactivated gauges (maybe cracked?) that become active when that type of round is implemented, and deactivated when its not.