high antimony alloy: how fast can I push it?

Hi Bidulph,

You are right, what you have is harder than linotype.   What's not generally known is that there are many “standard” lead/tin/antimony alloys.   Standard, that is, for specific purposes.  So you have linotype, monotype, foundry type, Lanston standard and others.   What we think of as linotype is 4% tin 12% antimony and is 22 BHN.   Pure lead is 5 BHN.   I'd guess that the stuff you have is about 25 BHN.   As linotype casts very well your stuff should too.    Should shoot well too, and 25 BHN is only one quarter the hardness of jacketed bullets.   So I'd be inclined to cast as is.   Just be aware that the higher the antimony content the more brittle the alloy, and it becomes quite frangible.  

Don't use it if you plan to hunt roos, you won't get humane kills, you might well get nasty surface wounds and condemn the animal to a slow death from infection.   The rule of thumb for cast hunting bullets is Lyman # 2 alloy which is about 15 BHN, but the same hardness can be got with 6 or 7% antimony and no tin.   At that sort of hardness it will penetrate and expand a little without fragmenting.   With gas checks it will not lead the barrel despite being quite soft at more than 2000FPS, but I can't tell you how much more than 2000FPS because I haven't tested it.

I am in the process of writing a book specifically about cast rifle bullets, so I am able to quote the results of my tests so far.   None of this is new information, but it is worth knowing nonetheless.   My tests are being fired in a Sako in 308 Win.   I have fired plain base bullets up to 1800FPS to test the MV limit of a handgun powder similar to Unique.   Chrono readings indicate that 1800 is pushing max.    At 1400FPS leading was virtually zero, and 1600 it was minimal, but at 1800 it was too heavy to be acceptable.   Antimony content was 12%, BHN similar to linotype.     

I tested the same bullets fitted with gas checks with a rifle powder similar to 3031.   The fastest I have pushed them is 2400FPS.   The bullet weighs 169 grains with gas check.   The primers indicate no excess pressure and the MV increase per grain has not started falling off, so I reckon I will easily get 2500FPS.   Leading is zero.

But take note that my tests thus far have been to establish the limits of leading, and what max MVs can be expected with various powders.   Until those tests are complete I shall not be looking for accuracy so I can't say whether velocities well over 2000FPS will be accurate.   I can say with confidence that you will get 2400 without difficulty with all three calibres with gas checked bullets.    Generally,  weight for weight, cast bullets can be driven 5% faster than jacketed at the same pressure, or same velocity at 5% less pressure.

But if you want to push max I would advise not pressing your luck without a chronograph.   Sometimes I feel the need to be cautious what I say to avoid teaching my grandmother to suck eggs, but for those who might not know, a chronograph tells us other things besides velocity.   One of them is warning of approaching max.   In my tests I am getting about 64FPS increase in MV for each grain increase in powder with powder similar to 3031.   It is a linear increase, but I know that the first warning will be an increase of 50FPS and when I get 30 or 40 that's where I'll stop and back off.   The importance is that when the MV increase slows, the pressure curve increases exponentially.   That's the value of a chrono.    I'm using a Chrony.   Very accurate and reasonably priced.  

If you want to reduce your alloy cost a little, you will find that 10% antimonial bullets will shoot quite well.   To get that mix 66% of your alloy with 33% pure lead.   Of course you are not likely to find pure lead at your local scrap yard but most scrap averages no more than 1% antimony so you can mix with scrap safely.   Don't worry about watering down the tin.   Tin is not needed for bullet alloy.

But for the long term why not buy scrap and blend your own alloy with antimony.   You'll still have to buy the antimony from a foundry or specialist metals supplier but that's the most economical way to get alloy and you have complete control of the mix.