He was worried about the amount of bullets he can put downrange. if he was a real gun nut, he would've gone for a simple blowback, open bolt sub-caliber weapon. Something that's simple to make, familiar and can fire cosecutively.
A WWII Emergency submachinegun such as a sten gun, m3 grease gun etc. would've been a better choice imo. easy to make, pistol caliber, and automatic.
It's probably better to make the AK after he master amount of magic he can put inside a rifle caliber bullet. but eh you know how power fantasy isekai works ¯\_(ツ)_/¯