i could use a .22 , a .223 remington, a .22-250, a .244 winchester or .25 winchester. I believe. sucks they cut if off at .25 caus my grandfather has a .25-06 id like to try on coyote or fox. but im just clarifying the caliber size and rule they set just caus. i gotta argue with guys in my club about it saying you cant use the .223 but im pretty sure the way they explain it you can. and im pretty knowledgeable with my artillery ha ha.