Can you get back in the magruder mine after story mode in gun

Yes you can!

I tried so many times to get into the mine but I couldn't go far. (I was only at the beginning that place where you blow the door.) But one time I was able to get into it. I am not completely sure how I did that, but I think It was like I rid as fast as I could to the entrance and then jumped from the horse and again mount the horse. It was like the horse went into the mine and when I mount it, it took me inside too. I even was able to go to the place where you fight the final boss. But I only did that once, but when I try it again, it simply doesn't work. Give it a shot and maybe you will make it too.