Hi John.
That's interesting. Antimony is found in most of the core metal fro most manufacturers, but 6% surprises me. I would have guessed it to be around 3.5%. I have been out of the bullet making business now since 2000, so some changes have probably been made in the last 17 years. I have a 1973 copy of Hornady's manual and their interlock bullets were all through it, but now I hear the lead cores are not the dame alloy as they were in the early 70s. I bought many thousands of the interlocks in the 70s, but I never tried to test their alloy back then, so I can only guess what the changes are that Hornady made from 1973 to today.
I do remember when Speer first brought out it's Grand Slam line, and the rear core was at 6.5%, but that was later changed. I suspect the whole core was then made with a high-antimony content. But what alloy I have no idea.
In eutectic lead based alloys the addition of antimony brings the hardness up but also the brittleness, so I would not think it wise to go above 6% ever in a rifle bullet. The addition of Tin can buffer that brittleness a small amount, but Tin actually is not for hardening as most men think. What Tin does it to make the alloy flow easier. In casting it acts as soap does to water. It makes the molten lead flow far easier and fill out the molds. In the process of swedging Tin adds a lubricity to the lead wire to make it form better. There is a down side however, in that tin can also cause the core to be slippery enough to make them more prone to jump their jackets on impact too.
Speer tried to solve that problem with it's "Hot-Core" line, but it's largely ineffective because their jackets are not tinned on the inside and no flux is used to cause the molten lead to bond. Cost wise, it's probably not something they can do.
I have done the same thing on custom made bullets to see if it will work and it does work wonderfully. What i did in 1994 was to take some stip and tin one side with 50-50 Bar solder and slick it off with steel wool. That left only a wash coat of solder on one side of the metal. Next I formed the cups with the tinned side being inside and poured lead cores into the jackets. The jackets were set on a piece of 1-1/4" bar stock heated to 750 degrees so the lead cores would stay molten for a few minutes and bond to the solder on the inside of the jackets. I then swedged the ogives and made the bullets ready to load. What I didn't do was to taper the jackets towards the tip and that would have been a lot better, but my experiment worked pretty well anyway. The strip I used was .028" thick so it was not too think to open up.
When fired at velocities as low as 1450 FPS from a 30-30 they still expanded well enough that I would have hunted with them and at velocities as fast as 3100 FPS form a 300 Win Mag, they shed only about 15% of the weight and held together very well. They were not ideal because of the lack of a thicker jacket with a taper of that jacket to .006" at the tip, but even knowing they were not perfect, they did a lot better than most commercial bullets I tested them against.
The down side was time... Such bullets, made right are very time consuming and could not be done with existing machinery at any price people could afford to pay.
I believe the next giant step forward in bullet will probably follow this lead or this basic idea. Cup and core bullets with thick enough jackets and thin enough noses would be very good and if they were bonded they would be super good. What the industry needs to do is to invent machines that can do it accurately and cheap. Today the hunter has a better selection of bullets than any hunter has had before in all history, but the technology has not come full circle to a point that such a bullet can be made for the price we need it to be for the average teenager to shoot barrels out of his rifles as we had in the 60s and 70s. If some company (I hope Speer and Sierra are listening) can come up with the machines to make this bullet at the cost of a standard cup and core, or maybe even cheaper, they would revolutionize the industry. It can't be done..................yet.
Flying machines, accurate semi-auto rifles and lap-top computers could not be done either...........until they were.
I have hopes that the engineers of tomorrow can and will come up with such machinery. I did write to Sierra about this very thing, but never heard back.
