I was loading for my cousin a few days ago and was using 140 rem core lok bullets. Best I could get was about a 10" pattern. Tried 4 different powders, 2 different primers and no change. So I started again where I began which was cci200, 50gr H4350 and this time used 140gr nosler bt ended up with a 3/4" group and was repeated. The rifle sure didn't like the remington bullet.
Your rifle isn't the only one. Years ago I bought a box of 500 Remington 150 grain .284" coreloks and tried to get them to shoot in 2 different 7x57's. 4 to 6" was about the best they would do and then a fellow by the handle of Seafire suggested I use 40 grains of IMR-3031. I gave it a try and the groups shrank to 1 1/4 to 1 1/2". Don't know why but, it worked for me.
Shew me thy ways, O LORD: teach me thy paths. "there are few better cartridges on Earth than the 7 x 57mm Mauser" "the .30 Springfield is light, accurate, penetrating, and has surprising stopping power"
