I kicked around the .45 acp too I have both calibers and can reload either. Thought the 9 would best..?I have the Vaquero in .45 with the acp cylinder. I like it a lot. No idea about groups because I always shoot it single handed but it is a lot of fun to shoot. How common are they in .357/9mm? I've seen several of the Blackhawk convertibles but not many vaquero convertibles in .357/9mm.
If I do the mounted shooting it would be just for fun at my house not only purpose gun or caliber.If you have thoughts about cowboy mounting shooting, you should check into which caliber they use. I thought they only use 45LC. The 45 LC and 45 ACP Vaquero might be your better choice.