I measured my PPS-43 bolt diameter at 1.134 inch. The mandrel should be that size diameter and the residual spring back would provide the needed clearance. Closest thing I can find is 1.124 inch diameter solid bar stock at the steel yard. A 4130 CMS flat will have lots of spring back so this mandrel should do the job.
I'm planning to use my adjustable AK jig set for this spec (ie 1.125 + 0.075 + 0.075). That 0.075 inch is about 13 guage sheet steel for my PPS-43. Those nice Polish kits use rather thick sheet metal construction.
The Soviet armorer and design books show the receivers as being 'steel 20' which cross references to 1020 or 1018 steel. They are not heat treated. 1018 steel should be easier to bend, less springy and less tough than 4130.