"Once you get up to that 18 BHN hardness, is that where the velocity can be pushed to the range of 1300 to 1600 fps?"
Just remember,, cast bullets are not jacketed. They do not perform like jacketed.
A harder bullet can be pushed faster. But to get a harder bullet,, as noted,, some tin can be added. But there is a point that additional tin is not helpful. Water quenching helps harden bullets, as does adding other stuff to the alloy. But again, there is a limit to what a cast bullet can be made to do.
I find it amusing when people think velocity is the key to things, and want to max load stuff.
Accuracy is #1. Followed by the performance of a selected bullet doing what it should should be the goal.
If paper punching is the goal, then accuracy is king, no matter the velocity. The DISTANCE added to the paper punching may necessitate the need for more speed.
Self defense comes down to what you desire a bullet to do. Are you trying to stop a Mack truck, or just a skinny thug? And what is beyond your target becomes a liability concern. All things to consider when building a SD load. (Yes,, many people say to not use reloads for SD,, but if that's what you have in your gun when confronted with a life or death situation you use what you've got!)
Hunting. This is where cast bullets can really shine.
But there are many, many variables to consider. First you match the caliber to the intended game. Then you match the bullet design to the game. Throw in the max range (distance) you'll attempt a shot. Add the fact that animals often do not present a perfect shot angle & presentation for shot placement.
Accuracy is king here, followed by bullet placement of the properly designed type. And it needs enough velocity to perform in the animals body to make a quick, humane kill.
Obviously a whitetail deer is a much different target over a moose or a grizzly bear, or heck, a cape buffalo. Each one requires a different approach to a clean kill.
My point to all this is that velocity is but ONE of several factors you need to consider when assembling ammo.
And for the OP,, do a search here on this Forum for a thread posted by "sixshot." His 2021 African safari. Handguns only, cast bullets, and their performance at it's finest. When you use a .45 Colt, and kill a cape buffalo (as well as a few other tough critters) quicker & better than many with high powered rifles,, you can understand why the combination of things must be fully considered. It's NOT just velocity. But cast can & does perform IF,, IF,, you do it right.
