Just got off a 20 minute conversation with Nosler. I called a friend who has worked at Nolser since the early 90's, he was busy so directed me over to another person (Mike) who is the Ballistic Tip guy, and was nice enough to answer my questions in detail.
I asked him about different jacket thickness in various BT's and how much they expand and how tough they are. He said jacket thickness is only a minor part of expansion, instead core hardness is the biggest factor in how much BT's expand, and every caliber and weight has a specific core hardness, and the hourglass shape of the core (see a cutaway of the Ballistic Tip), will vary to control expansion. The thickness of the base will also be specific to a bullet in each caliber and weight, and will be a factor in expansion. He also said the Ballistic Tip is not an elk bullet, and he does not recommend it for elk regardless of caliber or weight. He does recommend most of the Accubonds for elk, which he says expands slower than the BT.
Another tidbit I was not aware of is the Partition front and rear lead core are of a different hardness, and again the hardness of the two cores will vary with caliber and bullet weight, with jacket thickness being a small part of the expansion equation.
He was also careful to emphasized Noslers bullets are not drawn, but instead extruded. That's been discussed on here in detail before.
I have been given somewhat different information from Nosler over the past 30 years, though not from Mike:
Yes, core hardness is a major part of the performance of Ballistic Tips (as it is with many bullets), especially the first "Hunting" versions they brought out, which proved to be too expansive. This was because they were basically the same bullet as the Nosler Solid Base--but with a plastic tip. In my experience the Solid Bases were pretty tough bullets, and I used them in calibers from .224 to .30 on big game. But the tip changed things, because a BIG hollow-point was required to insert the tip.
Some of the early Ballistic Tip "Hunting" bullets were definitely OK. One was the 140 7mm, which I used in the 7x57 at around 2900 fps for years on deer-sized game, and never recovered one. But many were were not. The 115 .25 was one.... (The same thing happened when Hornady introduced the SST, but they fixed that as well.)
The heavy-jacket Ballistic Tips (which as mentioned earlier started with the 200-grain .338 in the early 1990s) were indeed much tougher, and the jacket was a LOT heavier. I first tested them in dry newspaper and they penetrated almost as deeply as the 210 Partition. Used them a lot in the .338 Winchester Magnum after that, and among other animals, one dropped a big gemsbok at around 150-175 yards. The bull stood quartering toward me, and the bullet broke both the right shoulder and the spine. Found it under the skin of the left ham, retaining 59.4% of its original weight. That was not surprising, since in the "heavy jacket" B-Tips the jacket is around 75% of the total weight. In fact, when the 200 .338 was first introduced, the late Chub Eastman said it kind of resembled a monolithic with a little bit of lead behind the plastic tip.
