Hook, from your description of the alloy, it sounds like there's a strong chance the spring in that Saeco tester might have weakened. Best to calibrate it against known pure lead (or close to it) for reference. Foster-style shotgun slugs and muzzle loader balls are convenient sources if you have them. 15 BHN sounds suspicious for a low-antimony alloy like your pipe fittings and wheel weight mix.
I don't know the meplat diameter of those bullets though?
