Red Dead Redemption Theory. Who trained Jack Marston in the ways of the gunslinger?

RDR, the game was a masterpiece. But, no matter how perfect it was, the big complaint I saw at the time of the game's release was "How did Jack learn to shoot in so little time." Well, I may just have a theory on that, and that theory was formed in the back of my mind when RDR2 first came out.

Every person who has played RDR2 knows the loveable character of Sadie Adler and the bond she shares with Arthur Morgan. Sadie Adler was a person who was hellbent on revenge and in the end achieved it. After certain events in the main game, we see the bond she once shared with Arthur, form with John Marston. Throughout the Epilogue we see their relationship develop (purely platonic). Trying to avoid spoilers, for the poor souls who haven't played the game yet, we see her set off on her own after the Epilogue ends.

The ending of RDR has got to be one of the saddest endings there is. When our loveable cowboy, John Marston, get's gunned down while giving his family time to get away. Years go by and we then take control of Jack. Jack, like Sadie, is on a mission for revenge. He's going to hunt down the no-good, son of a gun that betrayed and killed his father. And this brings me back to my theory. Jack is as skilled, if not more, than his father. However, the only time we see John "training" him was by taking him hunting. So what could have happened? Well, let me tell you my theory.

After hearing about John's death, Sadie spends months tracking down Jack and Abigail to ensure they're okay and to offer her protection to them. During her time with them, she saw the same desire for revenge in Jack that she once had and decided to train him. At first, it was just because she wanted to make sure he could survive out in the world. Then it became something else. Jack wanted payback. Eventually, feeling confident in the abilities and training she gave him, she left to go back off on her own. ( or maybe she died?) With only him there to protect his mother, Jack felt obligated to stay by her side until her last breath. Once she was laid to rest, there was nothing holding him back from hunting down and killing Edgar Ross.

Anyway, that's just my theory on the whole situation. I also posted this over on R/reddeadmysteries and got some pretty good feed back on it, so I posted it here!