From Dragon Ball Encyclopedia
Guns are deadly weapons that fire bullets or in some cases energy blasts. Guns are used most commonly by the people of Planet Earth. Apparently, guns have no effect against someone with a high enough battle power.
Just like their real world counterparts, guns come in many different sizes, shapes, and forms. In very early Dragon Ball, Bulma Brief shot Son Goku with a gun, only for it to have no effect on the young Saiyan – it did hurt him slightly. Many other enemies were seen using guns. These enemies mainly existed within the Red Ribbon Army, who used machine guns, shotguns, assault rifles, and even turrets to try and kill Goku, all of which have absolutely no effect.
The female character Launch also uses a machine gun on many occasions in her blond, more aggressive personality. She mainly used the gun on Turtle Hermit and Oolong. She was also seen using a shotgun when running from the police.
Police and the Earthling Army rely heavily on the use of guns, which they used against many of the villains (such as Piccolo Daimao, Nappa, Cell, and Majin Boo as prime examples) throughout the series.
On Planet Namek, Freeza's ground troops use a highly advanced type of gun commonly referred to as a Blaster. These Blasters were capable of much more damage to the common person as they shoot beams of energy. To people with enough power, however, they were completely useless. There was also another variant known as the Arm Cannon that fired an even more powerful beam (such as the one that Puipui carried).
During the Majin Buu Saga, two evil men named Van Zant and Smitty uses high-powered rifles to terrorize the streets. They eventually even shoot and nearly kill Bee, Majin Boo's puppy. They also shoot and nearly kill Mister Satan a short time later, the result of which sparks the creation of Evil Boo and the events following.