G19 is a compact in 9mm similar in size to the G23 which is the .40 cal version.
G20 is the 10mm full size frame.
G22 is the .40 cal. full size frame. (issued by a large number of law enforment agencies).
G26 is the sub compact in 9mm, which is same size frame as the G27 which is the .40 cal. version.
Check out this link! It's an older one and shows some older generation 2 frames, but you'll get the idea.
http://remtek.com/arms/glock/model/40/22/index.htmLone Wolf distributors handles a large supply of after market parts and accessories for the Glocks, and they have both factory and afermarket production barrels for your Glock.
I bought a 9mm drop in barrel for my G22, and all I do is replace the magazine with 9mm and it shoots without a hiccup!
I can also buy a .357 sig barrel and use the same .40 cal magazines.
I only do this for the range and only carry the original .40 cal. barrel and magazines for concealed carry purposes.
I would not take the chance and trust my life to anything other than the original factory parts!