assuming the barrel is clean, your not getting parallax error with your scope not being focused correctly @ the range, your not snatching the trigger or closing your eyes when you take the shot. Its not to hard to tell if its you or the gun which is at fault if you know you were 'on the target'
if you squeeze the trigger while cross hair is on the target AND your STILL on the target a second after the shot is fired (follow through) then you know the shot was on target and you did your bit then it could be a problem with the ammo/gun.
why not try shooting at 50 yards to start with.
mak