Why not the .40, since the longer barrel gets you more velocity? First of all, the .40 is disappearing. Yes, there is a huge established base of guns, and users, but they are not growing, and the 10mm is. So, future ammo production, and bullet development, is going to go to the 10, and not to the 40. Second, while the .40 does gain some velocity in a longer barrel, the 10mm already starts out faster than the .40 boost and goes up from there. You have to (or did, back when the .40 was a going thing) search diligently to find a .40 load that approached 1,000 fps with a 180-grain bullet. The 10mm does 1,100 to 1,200 with a 180 out of a pistol, and you can get…