You might have a hot load, but equally it might be (or also be) that your firing pin tips are in need of being reprofiled by a gunsmith. If their tip is damaged, the primer metal will tend to flow into it and the gun won't open. In my instance, I had to put the old double shotgun over my knee to get it open. Hot loads will definately exacerbate this. If this is the problem, you'll see drag marks on the empty primer/s. You might also use a magnifying glass, (I use a small jeweler's lupe which is quite handy for looking at things like muzzle crown damage, proof marks, primer craters, etc). If the pin tips are damaged, the magnifying glass will show it. You might also look at how far the pins protrude and also if their function is OK. A drop of oil might help too. Once I had the tips reprofiled, no more problems.

I have used various 7x57R loads by S&B, Norma and RWS with no issues in 2 different combination guns, the S&B copper-capped factory load in a Franz Sodia Bockbüchsflinte just 3 days ago.