Looks to be a Holland No. 2 Grade Hammerless LOG rifle in .500 3 1/4" Express. It's a black powder gun. The charge marked is the 136 grains of black that was normal for that cartridge, which most commonly used a 440 grain lead bullet. It's a useful cartridge, and pleasant to shoot.

The flip-up front sight is variously called a "moon" or "gloaming" bead. It's intended for use in low light. I've found them to work quite well.

"Serious rifles have two barrels, everything else just burns gunpowder."