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.