Como dizer "arma de chumbinho" em inglês

I believe the word you want is "pellet gun" (that would cover pellet rifles, pellet pistols, and pellet revolvers). The ammunition is soft lead. It is NOT a ball like a "BB". Common calibers include .177 and .22, but 5mm is manufactured by at least one company (Sheridan, I believe). Some pellets are propelled by compressed air and others by CO2 gas. I owned two Chinese pellet rifles that were easily the equals of .22 caliber rifles at 15m, and I have fired German pellet rifles certainly the equals of the average .22 firearm at 30m. BB guns are also "air rifles" but they are not "pellet rifles". The first air/pellet rifles appeared in the 1500s. By the 1700s some were used by the Austrian army. The first Giradoni air/pellet rifle was manufactured about 1790. It was in .46 caliber and could fire 20 pellets in around 30 seconds. At 90m the lead ball could penetrate 2.5cm of pine.