I like to ask why some bullets with a BHN of 10 or 12 I can scratch a line in them with my finger nail others with the same BHN is a little harder takes more then a finger nail to scratch them
I just got in a batch of 38-55 from RimRock bullet company 240 FN casted at a BHN 15
I loaded a few with IMR 3031 they shot excellent not as good a my sarco mould # 738
in 255 grains BHN 10 and 12 (yep these are soft lead ) no gas checks with 34.5 IMR 3031 and no barrel leading FPS high 1700
The Rimrock bullets looked real hard but yesterday a good friend has a lead harness tester and the Rimrock boolet came in at 12 from the readings on the lead tester.
I wonder if they add a different mix to make the surface of the bullet a little harder
Blending alloy in my experiences is part science and part voodoo. What you can see is several different alloys having the same or very similar BHN, but due to the composition of metals in them they will react entirely different. Such as with your fingernail scratching, one might be a pure lead and antimony and "feel" harder than one say of pure, with a little tin, and a little antimony or arsenic. Either tin, antimony, or arsenic will produce some hardening, but the latter two will go a step further than simply using tin alone. The tin acts like a binding agent for the antimony and the arsenic acts like a catalyst to kick the hardening into gear. It takes only a VERY small percentage of arsenic to jump things up several notches.
Also using an alloy with say a 1-3-96 percentage of tin/antimony/ lead, one can get a different hardness depending on how quick they were cooled, how quickly sized after they were poured, or at what temp they were poured. It all plays together to come up with the end result. I used a 1/3/06 alloy to pour up some rifles bullets not long ago. I air cooled half and water dropped the other half. I waited 3-4 days after before I had time to run them through the sizer and it took almost everything I had to do so. They got HARD let me tell you. Thing is, of the ones I DID size the ones which were quenched which started out harder on my tester, were now softer than the air cooled ones. Doing a bit of research I found that this isn't unheard of in that the outside was in fact case hardened if you will, where the air cooled were simply the same hardness to the core.
This could be what you saw with your bullets. Some were poured a bit bigger than the others requiring a bit more sizing which may have removed some of the outer hardness. Not saying that is it, but a possibility.
There is a much better explanation than I have for it here,
Cast Bullet Alloys , and a TON of info over at the other links on that site that dive in much deeper into it than I can put in one post.
I can only share what I have experienced. I have am relatively new to casting with only a few years under my belt, but most of this has been working with blending alloys in order to produce fully functional HP's that I can use for hunting or otherwise from my handguns.