Funny things air rifles . my old GC2 used to shoot 10.06 bisley mags but having shot 8.4s for a few years now ,it hates mags if i try to use em . you do not say what size group you are getting at say 45 yards , your max range ? i would try some hoppes NO 9 bore cleaner ( make sure it does not get into the internals ) and scrub the bore out with it using a pull through and tight patches til it comes through clean. my steyr takes 25 shot at least after cleaning to come back in , then i get a weekend shooting till it starts to tail off . luck HOLLY
PS having bought a rifle a few years back that would not shoot no matter what i did , in the end i got rid of it .
PSS if it is new then it could be barrel blacking or if secondhand maybe leading .worst case is rust , in which case US .