Make the end of the pin bigger
The cross pin for the rear sight is a solid pin and is relatively soft. You can pull the pin and while holding one end with pliers slightly flatten the other end by hitting it with a hammer against a hard metal surface. In so doing you will change its cross section profile from a circle to an oval. Only flatten the tip of the pin. If you flatten more than 1/8" you will restrict the ability of the sight assembly to rotate on the pin. Re-insert the pin, unaltered end first, from right to left. If you flattened it enough you should have to tap the last 1/8 inch in with a brass hammer. For a little insurance, put a drop of red loctite on the the tip of the pins and let it get sucked into the hole though capillary action. Wipe off any excess.