if I get a rifle that wont group first thing I check is the crown for nicks or burrs, then I get some fine polishing paste and a thin wooden dowel and wrap some soft cotton around end of stick and secure it with thread wrapped around the cotton material,
then apply some fine polishing paste to the cotton and then push the dowel in the barrel and work it in and out the full length of the barrel for about 10 mins, then I clean barrel with spirit or something similar and then push clean piece of cotton through, then fire rifle to see if it groups any better,
if not I repeat the process one more time, if it still doesn't group with any brand of pellets I try its time to strip rifle down and check other components like barrel bands and the likes.