I have owned and shot one since the early 90's. As pointed out above; it's easy to bend the loadinglever.
I ended up replacing mine with a solid lever and added a colt-style leverlatch too.
Start with a .451" roundball over about 20 grains of 3F.
I have also had good results with the .450" ogival pure lead bullet from a LEE mould over about 15 grains of 3F.
Note that the replica has a "pyramid" shaped frontsight, while the originals had a "pinhead" profile (=a small sphere on top of the pyramid). This usually helps bring the point-of-aim closer to the point-of-impact.
As to the shotgunbarrel: it is a lot of fun, and can be loaded with patched roundball, shot, buckshot, buck-and-ball, chainshot or a small stack of appropriate coins...! I normally use no more than 40 grains of 3F in the shotgunbarrel.
I have found that for me the LeMat doesn't really lend itself to "Duelist-style" -shooting because of the slanted grip angle and the corresponding hammerspur... it's more conveniently cocked with the aid of the weak-hand.
But it is one cool gun, and it has an interesting pedigree...!