If found this rolling block at a gun show. The seller did not know what it was, nor did I.
It is a No 5 or no 7 that was rebarreled to 38 special. Someone military tapered the barrel and hot blued it.
I worked up to 357 mag double loads with magnum small rifle primers. When the primer pieces, the gas tries to open the action and jams it for a few seconds.