In the first picture he is carrying a double barreled shotgun, and the pistol in his holster looks like a Schofield.

In the second picture, he has a Schofield.

First gun it's a Coachgun and the second one it's a Schofield

First one double barreled shot gun

Second one cattlemen revolver is my guess

Cuz when the game starts we dont get other revolvers only sawed off shotgun and cattelmen revlover so thts my guess

It's a Schofield, look at the shape of the gun.

Pump action Shotgun and Schofield revolver i believe, in my playtrough ive choose the Callaway Schofield revolver to be Arthur's primary and signature gun

That doesn't look like a Pump.

Yeah looks like a double barrel that's had the barrels cut down.