Whatever makes you happy, CW.
Bret, I'm working on it, actually BEEN working on it. This time I want to put together Basement Article #9, and have it peer reviewed and polished, put up for discussion, edited some more, and stuck in the read-only section for posterity. Here's the challenge, though: Dynamic Fit is actually at least a semester of graduate studies. I'm not kidding. If I put it in lecture form with all the chalkboard drawings and notes, the content would literally be like a full course in, say, binary phase graphs. Fiver could explain it in a few sentences, but only those who had already taken the course would fully understand what he meant, and those who are new to the concept wouldn't have a clue what he meant.
Alloy and BHN. Yes, well, you have to consider both. You need a certain toughness (BHN) to do certain things, but then you get into dynamics again and different alloy compositions having the same BHN (static) number will behave differently in motion. Water-dropped 50/50 alloy is hard, like 18-20 bhn after aging, but work-softens instantly when squeezed. Think of expanding spray foam, it's very rigid when cured, until you squish it and break down the internal bubble structure. It may spring back to its cast form when you let go, but it will be squishy throughout and not rigid anymore because you crushed the microstructure. The crystal lattice structure of lead (modified and refined through the addition of antimony, tin, copper, arsenic, etc) can be manipulated to do different things for....or against you. That subject alone is a six-week course, but is a super-critical part of the dynamic fit equation and choosing an alloy that will work best with a certain bullet nose shape and throat shape combination. The various behavior characteristics of similar-BHN alloys under stress and flow is why we can say use the MP 30 Silhouette in the .308 Winchester SAAMI throat with water-quenched 60/40 alloy and give it 20 thousandths distance off the lands, OR use the NOE 30 XCB bullet (different nose shape, doesn't self-align as well with jump) and seat it to touch the ball seat of the throat, but use an alloy that doesn't crush and break down as much under pressure (something near #2 alloy). The 30 Silhouette can be cast of a weaker alloy that draws better, sized really large to take up all that slack in the chamber neck to help guidance (IOW bigger than throat entrance), but still fold up like an umbrella (due to its shape and alloy composition) and draw down through the throat without creating excessive resistance that would cause powder pressure to bump too high and rivet the base before it gets into the throat.
You discover these things by trial and error, mostly. Like Fiver telling me to bump the tin for my .223 bullets and back off the lands a little as I increase the powder charge. Why did he say that? Well, if I hadn't backed off the lands at higher pressure, the bullet was getting hit too hard under too much resistance as I increased pressure/velocity and was starting to rivet and distort, ergo groups went to hell. Toughening the alloy and making it more resistant to breaking down by increasing tin to work with the existing level of antimony actually made the problem WORSE, but backing off the lands and letting the bullet have a head-start on the throat mitigated that. But when you back the bullet away from the lands/throat to give it that running start, the alignment is going to suffer because we don't live in a perfect world and something without full guidance is going to go wherever it wants to, so creating a tougher alloy (balanced tin and antimony, in this case, or it could be half a percent copper, or just powder coating what I had to toughen the skin) counteracted the tendency toward slight misalignment with the head-start/jump and allowed the bullet to handle banging into the ball seat in probably not a perfect, dead-straight alignment, correct itself, and get perfectly straight as it went into the bore. So I had to change two things at once to maintain the same group dispersion as I increased the powder charge. See how complex and inter-related this stuff is? How do I capture the essence of dynamic fit clearly in 1,000 words or less?
I learned a lot about alloy dynamics and fit from 44Man (James Miner). He wrote a lot about having a little wiggle to the cylinder at lockup, using water-quenched straight wheelweight alloy, and using a WFN or at least RFN bullet that did NOT have an abrupt diameter change between the base of the nose and the front driving band. He hated the "Keith" style SWC because all of the alignment force was put on that weak, abrupt leading edge and it would collapse rather than guide the bullet straight in the forcing cone. He needed a tough yet flexible alloy and a lot of surface area on the nose to take the stress of bumping the cylinder (and back half of the bullet) into perfect alignment as the bullet was transitioning into the barrel. If the bullet was slightly crooked and got smeared a little more on one side than the other, the base wasn't going to exit the muzzle perfectly squarely and the bullet would wobble in flight like a poorly thrown football (my analogy). This same concept applies to rifles, particularly autoloaders where we do not have the luxury of a hard jammed, fully-supported, no-tolerance static fit.