I bought a Navy Arms LeMat when it first came out. Here's my experience with it. After the bugs are worked out, it's a fantastic revolver.
The literature that came with it and all the gun writers stated to use .451 round ball.
I did that and the loading lever would bend on about the third loading. I contacted Navy Arms and sent it to them for repair. When I got it back, the loading lever would bend again.
Another call was made to Navy Arms. I explained to them that I was an experienced black powder shooter; especially with C&B revolvers. I sent the revolver back to them and when I got it back, I measured the chambers.
What I found was that each chamber was .444 inches right at the face of the cylinder. Now also keep in mind that when I took the measurements deeper into the chamber, the diameter got progressively smaller. I can't remember how much smaller but it definitely got smaller.
So right at the face of the cylinder there was .007 inches of lead getting shaved off. With a Colt or Remington revolver, that would not have been a problem.
The LeMat loading lever, on the other hand, is a steel sleeve with a steel rod inserted that is used for loading the shotgun barrel. So a LeMat loading lever isn't as strong as a Colt or Remington revolver. That's why it would bend with a .451 round ball.
So I switched to a .445 round ball. My load is 20 grains for FFFG with a 50/50 mix of beeswax and vegetable oil over the ball. Don't go over 20 grains.
I attempted to increase the charge to 25 grains and got a couple of chain fires. There were a couple of possible reasons for this.
1) I tried using the paper cartridges with the ends lubed with the beeswax/oil mixture. The amount of lubricant over the ball wasn't as much as if I had applied it using a finger or Popsicle stick. So there may have been less lube preventing a spark from entering the chamber.
2) The 5 grain increase was enough to place the ball closer to the cylinder face. With the slight imperfections in the ball, this may have been enough to create a gap for a spark to enter the chamber. Remember that the chambers have a slight taper. So a lighter charge would have resulted in a tighter fit because the ball would be seated deeper.
I went back to loading 20 grains of FFFG and applying a generous amount of lube over the ball. Since then, no more chain fires.
Some people may have had good experience using the recommended .451 round ball. I don't doubt what they say but I've put a lot of .445 round balls through the revolver. With the aforementioned problem as the exception, I haven't had any trouble with the LeMat.
As for the shotgun barrel, I use a 3/4 ounce load of shot. I use to just use paper for wadding but since I got a Pedersoli 20 gauge shotgun, I started using the card and felt wads, soaked in vegetable oil. You will find the recommended charge of 25 grains very anemic.
I use a powder charge of 40 grains in the barrel with 3/4 ounce of buck shot. Past about 10 yards the patterns are pretty wide. Perhaps other shooters can provide better recommendations for the shot load in the LeMat.
I would highly recommend the LeMat revolver. The only caution would be is to measure your chambers to determine what diameter ball to use before shooting it.