Steve,
Yep, 3.5% is probably just about the average percentage of antimony in cup-and-core bullets. Amounts varying from that are for specialized uses--but some bullet-makers aren't convinced that 100% weight retention is the ideal.
There have been a number of relatively recent attempts to produce inexpensive cup-and-core bullets that consistently hold together. About 15 years ago Federal tried with the Deep Shok bullet, by making the jacket thicker in the middle of the shank section (as Remington implied Core-Lokts were made) in order to hold the rear 2/3 of the bullet together. They worked very well, but unfortunately turned out to be more expensive to make than they anticipated, the reason they only lasted a couple of years on the market. (Core-Lokts actually just had a thicker jacket along the shank, but it wasn't tapered to lock the core in. More recent Core-Lokts are pretty much basic cup-and-cores, with thinner jackets to reduce production costs, or sometimes actually Interlocks.) The only Deep Shok I ever managed to recover from a big game animal retained over 90% of its weight--but they were gone not long afterward.
Right now one technique that seems to hold the most promise for relatively inexpensive bullets that hold together is electro-plating the jacket around the lead core, the process used in Speer Gold Dot handgun bullets and Federal Fusion rifle bullets. However, Speer tried to make their DeepCurl rifle bullets (basically the same as Fusions) at the same price as Hot-Cors and also failed. The big trick is varying the thickness of the jacket to get exactly the taper desired. Fusions hold together but tend to expand very widely, probably due to a combination of relatively thin jackets and soft cores. And of course to make the jackets thicker, the electro-plating takes longer, one problem in cost of production.
I have run into a lot of hunters who think Hot-Cores are bonded bullets, and even one or two gun writers, but as you note they are not, which can be easily proven by putting one in a vise, nose-up, then hacksawing it down the middle. The jacket can be just as easily pried away from the core as with any swaged cup-and-core. I've had Hot-Cors completely separate jacket and core even on deer.
