I have had many years experience with 7x57R in a HEYM, and my favorite ammo is RWS with 173gr H Mantle bullet.It was regulated with this bullet, but handloading with bullets as light as 162gr. worked well.Much lighter bullets, however didn't work too well.Individual groups were acceptable, it was just that they were centered too far from the Brenneke(other barrel)point of impact. Just a thought.
Mike