The hand guns in Red Dead Redemption seemed fairly well balanced in terms of what the various types had to offer, even if they didn't all share the performances of their real life counterparts/inspirations. Were it son, the Cattleman i.e. the Colt Single Action with its .45 Long Colt ammo, would have much improved stopping power, the High Power pistol would be a tad bit weaker, and the admittedly cool looking Volcaninc pistol would likely be the weakest of the bunch.
However, every shooter needs a hand cannon, no? Maybe not, and like I said the hand guns felt well balanced in Red Dead Redemption, so that the in-game High Power and Mauser sort of performed the task of a powerful hand gun nonetheless. Still, in an imagined Red Dead Redemption or a future Red Dead set around the same time, it would have been cool to see them include a pistol based on the rare Mars Automatic Pistol aka the Gabbet-Fairfax Mars. So writes the website Forgotten Weapons about this beast of a gun :
"The Mars pistol was a development of the turn of the century that would really be more at home in a Jules Verne novel (I would suggest the title “One Thousand Footpounds in a Handgun”) than in the real world. It was a massive handgun, ridiculously powerful, and marvelously complex. It lasted only for a short time, with the first production prototype made by Webley in 1898 and production ceasing by 1907.
(...) The Mars is 25% heavier and 30% larger than the Colt 1911, and fires a very similar projectile at 50% higher velocity. Its closest comparison today is the LAR Grizzly in .45 Winchester Magnum."
In other words, a true hand cannon, way ahead of its time at least in terms of power. A great, angry bear would be no match for Marston with one of these in his hands.
While tested, it was never taken into use by an army nor available on the civilian market, as far as I know. Only some 80 pistols were made, if I'm not mistaken, which would make it a true rare gun as well. One was recently put up for sale for a staggering $50,000 .
If not a future Red Dead then maybe another game set in around the turn of the century will pick up on the idea. You seemingly wouldn't really need to exaggerate its power, even.