One can build a projectile that will expand if “fired” from a slingshot. That doesn’t mean that it will penetrate adequately to be of any utility. My problem with nearly all the sub-sonic loads is that their effective operating envelope - driven by trajectory and adequate penetration make them of questionable practical value for hunting game animals. Even with vermin, I think we have a responsibility to kill quickly. Regardless of weight for caliber, an expanding sub-sonic load is challenged if trying to penetrate a boar’s shoulder except at very close range. Even in combat, a suppressed sub-sonic weapon is considered a tool for essentially pistol range applications.