No matter how much exertion Master Copper gives you, it gives you about twice times as many actions as normal firing.
For comparison, Burn. With the basic Coppertop that just loses you an exertion each time you fire, You Speed is five so you gain an action when your excertion is -3. So if you shoot at Mundanes 3 times, you get a bonus action for another shot- 4 shots in three actions.
With the Master level you have it at, you get two bonus Very Fast shots on Coppertops each action, though they gain you an exertion. You go to 5 exertion before losing an action. So, you fire three times in the first action. Then Three shots in the second action. Then two shots in the third, losing the main action from exertion. That is eight shots in three actions- more than twice as fast as everyone else!
So try not to think of it as gaining exertion, look at it as converting full actions into mere exertion- for you, an exertion is one fifth of an action.
If you don't have Lightning Reload, carrying spare guns instead of spare clips can indeed work well.
Ares, yes- with experience.