Well with a 1-10 twist you should be able to stabilize a bullet up to 220 gr. If your desire is to reach out to 1000 yards, your next investment needs to be reloading equipment. Since your rifle shot well with the one load there is probably nothing wrong with your rifle. I would seriously look at my scope and mounts if I were you. It is also possible that something has loosened up after all the 300 mag through it. Just a thought.Of course this would not explain key holing but it could explain flyers.