Leading because bullets are too hard can only happen if the bullet fit's loosely and allows blowby to strip the lube out of the grooves. So, if one has to shoot undersize bullets, making them soft enough so the base obturates inside the case, and causes a tight plug to seal off the blowby will prevent leading. However, hardness must be roughly balanced to the load so that obturation doesn't go too far up the bullet, increasing friction to the point that friction heat causes leading. In other words a balancing act which varies with bore condition, type of lube, and to some extent the type of powder being used, etc, etc.
With all that hassle to prevent leading, accuracy will still be inferior to what a well fitted bullet would produce, because the undersize bullet will remain undersize over most of it's length, allowing it to lean in the barrel and leave with a wobble.
When bullets are properly fitted, so the nose is a plug fit, or close to it, in revolvers, or in fixed barrels, so they have a plug fit soon as they start moving, the bullet will be able to maintain optimum balance, and deliver top accuracy. So, because the gas seal is assured, a good general rule would be: The harder the alloy the lower friction will be at speeds which are pushing the velocity limit. The other factors limiting velocity are then barrel temperature, and internal surface finish, and lubricant quality. The better the lube, the more any of the previous factors can be forgiven.
About gas cutting. It cannot happen until after the lube has failed, or limits of one of the above factors has been exceeded. The chain of events goes as follows. Normally minor leading will appear in the bore when obturation or velocity friction exceeds the strength of the lubricant, and this leading is picked off the very bottom driving band first, because obturation is highest there. Then, as rotational friction eats away the bit of lead on the driving side of the lands, a gap opens on the non driving side of the lands, which releaces hydrolic lube pressure from the grease grooves, and lets powder gas and lube to squirt through the tiny gaps behind each rifling lands. Presto, gas cutting. It cannot happen any other way. So gas cutting is never a problem but only evidence and the grand final of a problem. Read this over, ask more questions if you don't understand it clearly, and keep it in your mind, and you'll be able to wring the utmost out of plainbase bullets. Diddle around with trying to solve gas cutting and you'll get no where.
I'll give and example which prooves what I'm saying above. Something I learned by experimentation, of coarse, which included recovering spent bullets and scrutinizing for gas cutting under strong magnification.
Using an air cooled alloy, in 357 magnum, and testing with all the several different bullets lubes I was testing, leading, and gas cutting would occure at relitively low velocity, anywhere from 900 to 1100 fps. Using the same bullets, but changing only to LBT lube, I could run the pressures up till ejecting 6 cases at once from the double action required extreme force, with speeds around 1600 fps, no leading, and no gas cutting. I'm going to guess pressures were well over 50,000 psi, far above industry standard.
If you don't like scrubbing lead out of barrels, or cleaning them, try some LBT bullet lube, especially our blue soft. I recommend never cleaning the barrel, not even with a dry patch, when using it. I don't believe I've ever mentioned it in print, but all my lubes have an extreme water repellent additive in them, and it is by far the most concentrated in blue soft. The proportions of this agent are 1 part in commercial, 6 parts in blue and 16 parts in blue soft. The bore will not rust when it is left dirty after shooting this lube.
