18 bnh is about where bullets quench cast from wheelweight alloy land. I've shot thousands of them through .38s. .357s and .44 magnums. I've also shot quench cast wheelweight bullets through a 10" Thompson Center Contender in .357 Maximum in excess of 1700 fps and .303 British rifles at 1850 fps with no leading.
When I first started casting bullets I was concerned about barrel leading. But a proper hardcast bullet can be pushed very hard without leading the barrel. I used gas checked bullets in firearms where I was going to go much beyond 1150 fps.
Dead soft swaged lead bullets will lead barrels without being pushed very fast. But hardcast bullets never caused me any leading problems.
The hardness of the bullets isn't a help or a hindrance. It is what it is. The issue is bullet hardness along with bullet fit. I've seen more leading with ill-fitting hard bullets than ill-fitting soft bullets in revolvers, semi-autos, single-shots, and bolt guns. Hard bullets that are too small won't bump up under the relatively low pressure of 38 Special loads. This allows gas flow around the sides of the bullet, which causes leading, specifically along the leeward side of the lands. I've seen it dozens of times in others' guns. Hardcast bullets are far harder to get to shoot, unless they are cast to fit a specific gun's throat dimensions.
Well,..that's not been my experience. Most of the molds I cast from would be about .001" oversize as they dropped from the mold. I never sized them. I would run a sizer/lube die .001" over to apply the lube and attach the gas check. I never had any leading problems with any of them.
I competed for a couple of years in IHMSA using nothing but my own cast bullets made from quench cast wheelweight alloy. I shot in AAA class in both production and revolver.
Currently, I'm shooting both hardcast and swaged bullets out of a collection of K-frame .38 revolvers. The swaged bullets will start leaving small streaks in the barrel at around 1000 fps. But I push the hardcast bullets as fast as I happen to feel like with no thought to leading the barrel,...been that way for a long time.
