Yeah, 12.5's are pretty "primitive" too, and I wouldn't want to get in the way of one of those.
(?See the new Sherlock Holmes movie. Morriarty made the comment, "That's not fair" when Watson deployed a "primitive" weapon - a cannon. Proving once again that bigger is better, until you get to biggest - and if the BG has it, time to run fast.)
Biker, there is a guy up in Alaska that makes a neat holster for big hunting guns. I have one for my Desert Eagle. It is designed to be a vertical "shoulder" holster, but actually hangs in front of you. AND it's designed to be worn OUTSIDE your jacket, so it is readily available. On a horse it just might be the thing.
Guy's name is David Johnson. For orders, his phone number is: 907-688-0366. You can send him an e-mail at: firstname.lastname@example.org
This is not a concealment holster but a practical working one, made for the outdoors. Good workmanship, carries a big gun like a DE well, and is instantly available right in front of you. I wear it on my left front when I go hunting, in case I put my rifle down for some reason and need to grab a weapon quickly. There seems to be plenty of adjustment for "little guys" like me.