@Random commenter
He's using the Black Beth which is not a Musket but rather a Sniper Rifle as shown multiple times. It also has been shown that the Rounds he uses are not only Bullets rather than Ba*** but rather, he uses actual Magazines, so for someone like him who was a Highly Skilled Sniper, Combat Trained in the Army, he should be capable of actively reloading a Mag within Ten Seconds and that's being generous, Three to Five Seconds would be more likely.