I can tell you from experience, I have shot Hornady Customs in my .257 Roberts and 6.5x55, both loads shot very well sub moa if I did my part. The only light magnums I have shot have been for my 06, that was when I was still thinking I would be taking the longer shots on a regular basis. You don' t say what your twist rate or length of your barrel, both factors that can really affect performance. With my 06, 24" barrel as I recall the average was about 1.25" - 1.75" @ 100 yds. Sounds like your gun just don' t like them. It happens.