Take a small center punch. Cut a piece of conduit (12" to 18"). Drop punch through conduit onto nose of cast bullet. Measure the "divot".
Or...buy the inexpensive Lee Tool.
Plumber's Lead is closer to pure lead. It will be more suitable for Black Powder work. Can always add a little lead to wheel weights for .38 & .45acp target bullets. They work best a bit on the "soft" side.
Hard bullets can, at low velocity, lead worse than SOFT bullets.
I don't think a Plumber uses Babbit, plus it's very expensive. Babbit is used as bearing material in old machinery, tractors, freight elevators in old factories. If you have soft lead add some 50/50 Solder or wheel weights. I use 9 pounds of wheel weights to 1 pound of 50/50 solder to cast bullets.
Get the Lee tester. It is used on any single stage reloading press made for standard dies. Mine cost me $50.
I use recovered range lead and wheel weights and usually get BHN below 15. Actually prefer to se it around 12 or lower.
Have you delved into the Cast Boolit forum? Cast Boolits
it contains a wealth of information on casting and shooting your own.
I've been reading all the various posts about methods and devices for testing alloy hardness. One post by Molly recommended using pencils of varying hardness to compare against the surface hardness of bullets and ingots. This is widely used in labs to test hardness of metal coatings by...
Lead Hardness Testing using the Staedtler scale
Staedtler Hardness Chart:
8B Sheet lead
6B Lead wire 5 BHN
5B 40/1 lead / tin
3B 1/20 tin/lead 10 BHN
3B Clip-on WW
2B Range scrap
B 20/1 Lead / tin
B Quenched range scrap
B Air cooled WW
B WW+2% tin
HB Lyman #2 15 Bhn
F Lyman #2
F Commercial cast
H 50/50 Lino /WW
H Linotype 20 Bhn
2H Quenched WW
Rough BHN to Staedtler Hardness Conversion Chart:
6B = Pure lead, about 5 BHN
3B = 1in20 tin/lead alloy, age softened, about 10 BHN
HB = Lyman no 2 alloy, about 15 BHN
H = Linotype, supposedly about 22 BHN, but that seems high