The Fosbery bullets are best launched at just under the speed of sound. Otherwise, the bullets will go trans-sonic in flight and become erratic. At subsonic speeds the Fosbery bullet acts very much the same as a pistol bullet. I once worked up the ballistic coefficient by chronographing them at the muzzle and then again down range and I was quite surprised. The BC was much higher than I expected. But once I compared it to BCs of other big cast bullets at the same velocity I realized it was right in there with them.

The original alloy was 15:1 lead to tin. Later, Kynoch at least was making them with lead and 2% antimony. You can look up the hardness of that mix but suffice to say it is still pretty soft. For the short time I was casting them and making my own ammunition I used the readily available 16:1 mix and it worked just fine. The current factory ammunition is loaded with a lead-antimony bullet. I bet they are following the old 2% formula Kynoch used but I cannot confirm that.