Caliber? I have one in .22. Seems to like H&N FTT's 5.53 head size and actually doesn't do too bad With the Crosman's. JSB 15.89gr are pretty good too. Wasn't expecting much for a $60 refurb from Midway but it's not too bad a rifle and it now wears wood since I had a Benji Titan thumbhole stock just sitting.