Leading is caused by trying to dive any cast bullet too fast. It's friction that melts the lead not the burning powder.
A number of factors can contribute to leading.
I have already described shooting full power wheel weight bullets (approx 250 gr) ,non gas checked,out of my Super Black Hawk.Using H-110
I can't give a measured chrono fps,but the load manual says well over 1400 fps. Given the revolver has a cyl gap,I'd adjust that down some. I wil claim over 1300 fps. No leading,no gas check,BUT, bullets used are .430/.431 .They are not undersize.
My brother has an early Kimber 1911,from the days when many regarded the Kimber to resemble a Colt 1911 after a good pistolsmith worked over the Colt .
It has served him well for probably 20 years.
I convinced him to try shooting LaserCast 200 gr SWC's for target shooting.
He got leading just ahead of the forcing cone. LaserCast bullets are quite hard,and these were target loads around 800 fps.
So,what gives? The 45 cal bullets were not large enough OD. And,being very hard,would not "bump up" to seal the bore.(Which may or may not happen with softer bullets).
Anecdotal? Yes. But it illustrates that there is more to leading than velocity or hardness.
A loading bench error resulted in me shooting a few 308 loads that were loaded with pistol primers. My habit of looking at primers shooting handloads noticed pierced primers pretty quick. I shut down shooting them within 10 rounds. Under magnification,I could see firing pin erosion had already started. If you have experience leaky primer pockets,you know how fast the gas can cut the hard steel of a bolt face,
Gas will cut the top strap of a revolver.
And gas leaking past a cast bullet will cause leading.IMO,that is the primary cause of leading.
See if this illustrates. Top smallbore competitors,shooting very expensive rifles with custom barrels...What might their cleaning routine be?
Many of them don't run a cleaning rod through their barrels at all,and they darn sure don't use an aggressive scrubbing routine to remove non-existant leading.
Yet they shoot soft lead bullets,no gas check,target loads typically are not plated,and velocity is generally 1000 to 1100 fps for standard velocity target loads.
And the small amount of dry lube has to do for a relatively long barrel.