After a short while into Red Dead Redemption 2’s story, Arthur was given the ability to wield two handguns. The first (your main holster) sits at your right hip, while the second sits across Arthur’s belt line, just below his stomach (Off-Hand Sidearm).
However, some of you (like I found) may have noticed that your main (hip) weapon can get replaced with a different weapon that you’ve been handed or have picked-up. This sucks at first, as the game doesn’t tell you how to swap the guns back over and if you’ve spent some time customising your main revolver/pistol, you want to make sure that baby gets holstered in style (double tapping L1 makes Arthur holster his main hip weapon in sweet style!).
In order to switch handguns to different holsters, go into any Gunsmith and choose to customise a weapon. You’ll now see a list (as in the image above) of all your available weapons, as well as a symbol next to whatever is equipped in your main hip or off-hand holster. You can simply select the gun you wish to move and press the related button (displayed on the menu to the bottom right of the screen) to swap your sidearms around.