My last 91, I just glued some sheet rubber to the side and peeled it off when I sold it.
Same for the 770.
Steel case, not fond of in the gun.
The brass is supposed to expand into the flutes, then the gas pushes back to release the case. Steel doesn’t have the same spring back that brass does, it seems to lock in harder.
At the same time, some commercial brass can be too soft and will have issues if you choose the wrong powder, the case will still be adhering to the chamber walls when the extractor is trying to extract the case. You end up with a really bent rim.
Part two, the bolt face will leave a ring around the primer, it’s designed for the case head to set back, creating a seal to prevent primer loss or leakage.
You can easily remove this with a chamfer tool, go easy you only have to do it the first time.
Funny thing, my old 770, same load, fresh brass vs once fired, the SD went from 15 to 7.
For the best results, stick to 7.62×51 NATO loading data regarding powder, primers and pressure.
Commercial load data and pressure designed for bolt guns can give issues.
Avoid Hornady “Light Magnum” loads.