I use the rotometals alloy. But I think you misread my question (or maybe I don't understand your answer). I'm not looking for academic information. I'm looking for real-world experiences with an analog to Bismuth roundball -- hardcast lead. There's experience-a-plenty with pure lead for hunting. But pure lead expands. Seems that it expands to about quarter-sized a lot of the time. That's a very different dynamic than hardcast, which is all punch and no expansion. Bismuth alloy will absolutely punch through whatever critter it hits -- no question there in my mind.I'm confused, are you shooting 100% Bismuth, which isn't an alloy, or are you shooting something like a Bismuth/Tin Alloy? Rotometals sells Bismuth/Tin alloy in a 58/42 Bismuth/Tin, 55.5/44.5 Bismuth/Tin, and 40/60 Bismuth/Tin ratios.
The simple answer to your question would be to take a scale and weigh the ball that you're using. Compare it's weight to a known quantity, such as an all lead round ball of similar diameter. Then decide. For example, a .440 lead round ball is about 128 grains, but probably not what you want on an elk or bear. A .490 round ball weighs about 177 grains and would work on Elk and Bear... so where does your ball fall between those two weights? You really can't count on lead round ball deforming a significant amount, as the sphere sheds velocity so very fast in flight. A lighter projectile will start out faster than the all lead ball, BUT it will also slow down inside the animal faster, due to less mass than the all lead ball, but nearly the same friction. So as you have found, worry on shot placement, and original diameter.
LD
I've had 2 1-shot kills with roundball -- one with a bismuth ball and one with hardcast lead. Very different shots though. The bismuth ball was a frontal shot at about 15 yards on a deer in the lower neck area. Deer dropped immediately & was dead in seconds. I recovered the ball down in the femur while butchering -- hardly a scratch on it. The hardcast lead was on a 50-lb javelina broadside punched perfectly into the heart & clean through at about 60 yards. That pig hardly knew he was hit -- ran at the shot but not far. He just needed time to lose some blood in his brain before he collapsed. Thankfully, he didn't go far. But there was no blood trail at all. Granted, there was hardly any time for the blood to pool up inside & start leaking out. Especially since his heart was wrecked & couldn't pump anymore. But had it been a liver shot or something less than ideal, I don't know if I would have been able to recover the animal. Heck, it took me several minutes to find him where he laid even though I watched him go down.
So what happens if I take this up a notch & chase elk or larger bears (I get opportunities sometimes) with a .50? Am I likely to give up a blood trail? Should I be stepping up to a larger caliber like a .54, .58 or .62? Or is the .50 still an adequate diameter?