How do bullet manufacturers vary core toughness in bullets so that varmint bullets go poof and larger game bullets push in? Or is lead just lead and they are kidding the Biscuit?

Heat treating. They have a lead hardness scale according to some posters on here. Now how Speer have two different cores with grandslam, I don't have a clue how they do that.

lead is alloyed with (up to) 6% antimony to give it extra hardness. Some bullets are made with pure lead and some with antimony alloyed with it. There's no heat treating with bullet lead.

Iunderstand quite a few bullet manufacturers use pure lead - and allow the jacket engineering to control the degree of expansion. I know a few Sierra bullets use a small amount of antimony in the lead cores - like some of the heavy weight roundnose pills. And I understand that Speer uses two different lead mixtures in the Grand Slam bullet. But I believe most bullets, either varmint or big game, have a pure lead core.

Probably more important than the hardness of the bullet core is the design of the jacket (to which someone previously alluded). The thickness and profile of the jacket is the thing that controls expansion. For a varmint bullet, a relatively thin jacket, in combination with the action of the "ballistic tip", causes rapid, violent expansion (and often, fragmentation). Varmint bullets also depend on high-velocity, to help this process. For a bullet designed for heavier game, a thicker, tapered jacket design, thicker at the base....and often "bonded" to the core by mechanical or chemical means, causes the bullet to "mushroom"....but still largely remain whole. Whether or not a "ballistic tip" is employed in heavier-game bullets...the action (and design) is basically the same. Of course, there are several differing designs of this sort of bullet, including the compartmented types, such as the Nosler Partition and similar Swift A-frame...but in each case, the jacket controls the degree of expansion (and whether the bullet will fragment).

I know of no jacketed bullets that use heat-treated lead alloys in their cores, but it is not impossible to do this. However, heat-treating is quite common for cast lead bullets.