Need more info to be sure, but if there isn't any pattern at all, like it puts one round on center target, the next is on the top right corner, the third is lower right, fourth is an inch above canter, and hitting far left, then we can narrow it down to a couple things.
1) you can't shoot for hell
2) its the scope/mounting
3) you've accuired Franken-rifle, and the makers just cobbled together an assortment of Ill-fitting parts, stuck the trigger assembly from a Mossberg shotgun in there for good measure, and packaged it up as a 770.