What is the difference between the 9mm and the 40 caliber? And since I already have a 9mm would adding a 40 caliber mean very much?
Besides the obvious difference in size?
The .40 is intermediate between 9mm and .45 ACP, in pretty much every way. Guns that shoot 9 will most often be available in .40 as well, as little redesign needs to be done.
You can shoot a smaller gun comfortably, in 9mm, than you can in .40. Gunboard folks, may of them, tend to complain about "snappy" recoil. Recoil sensitivity is an individual thing
. I thought .40 was a bit much in the Springfield EMP I tried once. Above that, not so bad. "If
you have raced with men on foot, and they have wearied you, how will you compete with horses?"
If you already have a 9mm, adding a .40 probably won't do that much extra for you.