Excellent question. One thing to note is that the Barretta M9 is the standard issue for the military so perhaps he is in the military or just a casual fan.
As far as 45mag, does that exist? I'm aware of 44mag and 45acp. Both of which are more powerful than a 9mm and would have more recoil and less shots on target in theory. The ammo for those big guns are also much more expensive than 9mm.
38 special is weaker than 9 and it is also not an auto ammo, it is used in revolvers. I am not sure of the cost of .38 special but I imagine it's close to 9mm but not certain.
.38 cost depends a lot on the brand (tarus, s&w, exp...) You can get a Charter Arms 38 for less than $80. Better off getting a .357mag and shooting .38s out of it for practice (they'll fit)
.38s are garbage though.
Fun guns? Desert Eagle .50 A.E. or a double barrel 1911 ( they exist.$ for about $9k)
Justbplease god don't waste any money on a Hi-Point 9/380 or a Brayco. Lol.... Actually, I had a Hi-Point .40 tact carbine that was actually pretty good for the $180 I paid for it. Had a beam, flashlight and hologram.. 5" groups at over 50yes ain't bad for home defense. Plus they look intimidating
Norinco makes an Uzi with a 16" barrel & seminauto (for legality) for about $280... Home envader will th I k twice if they see it. Lol.
If u go ar15 if you go with a C3 11.5" barrel you loose the fps those rounds need to be effective. U can get away with it on an ak. (Course those all need SBR licenses)