Hi there fellow Heym 7x57 owner, do you buy or home load your ammo? I have been the proud owner of a Heym Trophy 7x57 for over 12 years and it is without doubt one of the finest all round calibers around, the 6.5x55 swedish comes close though, if you are buying your ammo stick with the 139,140 or 150 grain bullets, Norma, Sako or Lapua brands are the best, plus if you save the brass you will have a top case to home load with, if you are a home loader the same applies 139,140 or 150 grain bullets, my rifle shoots sub 1/2'' groups at 100yards with home loads with a muzzle velocity around 2600 ft per second, if you load much faster the accuracy will suffer, one last very important tip, watch out for copper build up in the barrel, there are a lot of gun cleaners that claim to remove the copper fouling, most don't, I have found Sweet's the most effective.
Good luck with your new Heym, I'm sure that you will never want to part with it.