I built a backstop in my yard for shooting cast boolits. Having cast and shot a couple thousand, and having recovered several hundred of those, these are some things I've noticed:

Very few slugs are demonstrating any expansion. This is true across velocity levels and calibers. Maybe 5% of recovered projectiles are mushrooming to any extent. Many of the recovered boolits are deformed to a greater or lesser degree, though much of this is due to hitting obstructions in the backstop (nails, knots) I assume.

On the plus side, virtually all are intact meaning that no significant weight loss occurred and very few broke into two or more pieces.

Not surprisingly, all the gas checks are shed early in the backstop and I can't recall recovering a single boolit with its gas check still attached.

Across velocities and calibers, no keyholing or unstable flight paths are occurring. Although I'm not getting the one hole accuracy claimed by many with cast boolits, this is encouraging me to keep at it and keep experimenting with components until I get better results.

Penetration can be quite good with the heavy or high speed boolits. Many pistol projectiles were penetrating many or even all of the layers of the backstop even at moderate velocities.

I'll report again in a few months when I rebuild the backstop and recover some more results.