Get yourself to Benedict Point (using any method you see fit) and you’ll bump into your old mate Irish again. After a heated exchange of words, get on your horse and saunter along towards Gaptooth Breach .
When you reach the entrance of the camp site, switch to your Dead Eye and use it on both of the guards that are on the horses in front of you. When you get closer, all hell will break loose and a bunch of miners with weapons will start firing at you!
Stick close to cover and use your long-ranged rifles to pick each of them off one at a time. If you haven’t got a good shot, use the cover to flank the enemy and get a better shooting position on them.
Now get into the mine and keep your eyes open for numerous TNT crates they’ve foolishly left lying around the tunnels! Shooting these will immediately kill anyone nearby, saving you time and ammo. Stay slow and low; and also shoot down any lanterns from the roof to burn those red blips off of your radar!
Work your way down to the mine cart and press Y/Triangle to start pushing it all the way back up through the mine until you reach the yellow cross. After the cut-scene, push the cart downhill so it picks up speed and you’ll automatically grab the cart and ride it down!
However, watch out for a barrel of TNT that’s dropped on the track at the bottom of a hill, shoot it out of your way ASAP! Once you reach your destination you’ll complete this lengthy mission.