I have never fire a muzzler, I've seen them fired at exhibitions.
I've read that British troops in the Napoleonic Wars could fire 4 rounds per minute, but the less well trained French conscripts could manage only 3, at best. Range was about 60 yards. The Brits would march to within 60 yards and start volley fire with devastating effect. Few opponents could withstand that.
The French would often move in a solid block so if you killed the front rank they would appear not to have changed, and poorly trained troops would nearly always break. The British (and many Prussian troops) would stand in line and return fire. The tactics of that era are fascinating to me, including the "make square" that they could manage in short order.