It's a common trope in first-person shooter video games for the stopping power of a firearm to be inversely proportional to its fire rate and/or magazine capacity. This is done for game balance, in a world in which pretty much anyone can choose freely between a handgun, a sniper rifle, and a squad machine gun as their personal weapon. In reality, it doesn't really work this way. Someone hit by a single 45ACP bullet shot by a Thompson Submachine Gun (Tommy Gun) will probably sustain at least as much tissue damage as someone hit with the same bullet fired from a 45 caliber handgun such as an M1911. The reason that slower-firing weapons are not obsolete in our world is due to practical aspects - a large, lumbering, high-capacity, fully automatic weapon costs more to make, maintain, and supply than a cheap handgun or a basic infantry rifle.
What differences in technology or environment would cause the video game trope to become true? That is, what would make armies intentionally choose to equip their soldiers with pistols, bolt-action rifles, and small-magazine automatics because they are tired of using up an entire belt of MG-42 ammo just to knock out a single enemy infantry soldier?