$500 - $550 new in box, give or take.
9mm
Glock is a brand. 9mm is a caliber size. So, there is no difference between a 9mm glock and a 9mm of another brand as far as caliber size.
No. The 9mm is not compatible with the .40 caliber frame/slide. On the other hand, you can put a .357 SIG barrel into a Glock 22 and use your .40 magazines.
The model 19 is a 9mm only proposition. The model 19 is not available in .40 caliber, but the model 23 is the same size gun and it is a .40 caliber gun. Some people would say that the model 23 is the .40 caliber version of the model 19.
100-500 or so
He got shot with a 9mm semi - automatic pistol
The .40 caliber fires a larger, more powerful cartridge than the 9mm. which is about .35 caliber. Minor additional info: Many models of pistols that are available in 9mm or .40 are otherwise the same, except for caliber. For instance, a Glock 19 and a Glock 23 are the same exact guns, but the 19 is a 9mm and the 23 is a 40...the 23 holds 2 less rounds.
Yes.
515 USD
No. Because a Glock 26 is chambered in 9mm, as a Glock 23 is chambered in .40 s&w. For example you can use a Glock 22 (full size) in Glock 23 (compact) and in Glock 27 (sub-compact) But you cannot use Glock 27 mags in Glock 23 or Glock 22.
Glock 9MM Model's 17/19
Yes, a Glock 19 9mm magazine can be used on a Glock 23 since both firearms are chambered in 9mm.