Ok, a .5" group isn't that big of a group. I have been shooting out to 1130 yards with my half inch load lately. I blogged it all on video and even though its the same old prattle from me, I recommend you watch it:
YouTube - Ballisticstudies's Channel
There will always be a number of variables that may cause fliers. I would like to suggest only one variable out of the many- please consider it carefully. You are shooting a magnum powered cartridge and as such, your shooting technique has a huge effect on group potential. Human error is the first thing that needs to be removed from the equation. Again, I talk about my own troubles with this in the above video clip. I went into great detail in this article:
Hold that Forend!
The slightest inconsistencies or mis-understood techniques will produce the results you are experiencing. I am not saying this 'is' the cause of your fliers. Its just one variable that has to be removed and something we all need to monitor, all of the time. Once that is out of the way, you can concentrate on other variables such as barrel heat/ steel stresses, pinch points in the bedding, fouling and bore condition, bullet to bore concentricity, extreme velocity spreads.
If I had to make a guess (which is all I can do without testing the rifle), based on the work I do with client rifles on a week to week basis, if a sub quarter inch rifle produces small fliers of 1 in 6, it will be you, not the rifle. Just bare in mind, its only a guess/assumption. I am pretty confident though.....
Hope that helps give you a starting point,