It all depends on the bore size, and if you really mean GUN as in "gun or rifle." The larges Pellet bore is 50 caliber and that would be a Air Venturi .50 Cal, 275 Grains, Solid Core, Soft Lead, Round Nose pellet.
If it's just a pistol it would be a .22 caliber pellet and that would most likely be a Predator Big Boy Senior .22 cal, 28.5 Grains, Domed.
Remember air gun/rifle pellets come in all sizes. .177, .20, .22, .25, .9mm, 357mm, .45cal and 50 Cal.
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