I was at the range yesterday with my brother-in-law and had this idea, just wondering if I'm on to something or if this is just a shower thought. He carries and shoots a .40. I carry and shoot a 9mm. He's a little less experienced than me and his groups tend to show that. After shooting for a while, we traded guns. He typically shoots a Whalter PPX but had out his XD-S .40, while I was shooting my G19 and Shield. When he got the 19 in his hand he was banging out groups on par with the best that I've ever shot. I attributed that to the 19 being a larger gun with more grip and easier to control, but then he backed it up with the Shield. Now, I'm a firm believer that you should train with what you carry, both in the model of gun and caliber. But, he was going on and on about how he thought he was more accurate because it was a 9mm and not so much because of differences in the gun. To back that up, I did have him rent a G23 and G27 because I've got him curious about buying a Glock. His groups with either of those were about on par with what he was shooting with his XD-S and PPX. So, it got me thinking that maybe he's right and it has to do with caliber. Maybe he's anticipating recoil, etc. It started to make sense to me that if you're used to firing a hand canon, then in the heat of a self defense situation you were carrying and firing a lighter firing caliber, you might be more accurate with it. Does this make sense or no? It doesn't seem very practical to own two guns just for this reason, and it's probably more logical to just stick with the round you're more comfortable.