If it's a brand new rifle, there can be a breakin period with some barrels or it may just be a case of trying a new load. Some rifles, however, don't reach their full potential until they have had a 100 or so rounds run through them. I don't think it's a matter of calibre but more likely the barrel. There are a thousand variables that will affect accuracy before you get down to comparing the inherant accuracy of certain calibres versus others.