Originally Posted by BB57

Yes, but it's complicated.

You want to match the pressure of the load to the alloy hardness. If the alloy is too soft for the pressure, it will obturate just fine, but you'll get some blow by and gas cutting, which will leave lead in the bore. In extreme cases, the bullet will "strip" in the rifling and you'll get both horrible leading and a bullet that won't spin enough to stabilize.

On the other hand, if the alloy is too hard, it will not obturate enough or soon enough in the bore and you'll get gas cutting and leading in the bore as the gas goes around the bullet.

Generally speaking using an alloy that is too hard os more common than using one that is too soft.

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Plated bullets usually have a very soft nearly pure lead core with a thin copper plating that really functions much like the lube on a cast or swaged lead bullet.

You can find heavy plated bullets that are designed for 1500-1800 fps, but they are usually for cartridges like the .30 carbine or .30-30 Win. Most regular thin plated bullets are not intended for more than about 1200-1250 fps - and that's only about 100-150 fps faster than what you can do with a soft swaged lead bullet using a good lubricant.

That 100-150 fps however can be significant. I'd love to see a thin plated 158 gr LSWCHP with a pure lead core as I could then launch them at around 1150-1200 fps and get excellent expansion without the leading issues that can be an issue with a swaged lead LSWCHP.

You can certainly drive harder lead alloys (like Lyman No 2, which
at a bhn of 15 is close to ideal for .357 Magnum pressures, or a slightly softer 1-20 tin/lead alloy with a bhn of around 10-11 which is great for mid range .357 magnum loads) faster, but then you run into expansion issues.

When I used to cast bullets, I'd use 9 pounds of clip on wheel weights alloyed with 1 pound of 50/50 bar solder to get an alloy close to Lyman No 2 at low cost. 1.5% to 2% tin content in the alloy also helps reduce the dross and gives you a much nicer fill in the mold.

Bullet diameter also makes a huge difference in terms of avoiding leading.

For example, I worked up a swaged soft lead158 gr LSWCHP load for my 2 1/2" Model 66 and 2 1/2" 686+ at around 1100 fps. The load shot very well, with excellent accuracy, excellent expansion and penetration, but it took some work to avoid leading issues. I ended up having to lube them and then tumble lube them in liquid Alox to be able to put 50 rounds down range with no leading.

However, when I bought a 3" 686+, the first time I tried the load in it, I had bullets going through the target sideways at 7 yards. The cylinder and bore dimensions were enough larget that the bullet just wasn't obturating enough in the bore to adequately engage the rifling, and it needed a larger diameter bullet. Basically it was the difference between a cylinder's chambers being cut with a new reamer to larger dimensions than occurs with an old reamer.

Generally speaking you want a bullet sized to fit the dimensions of the leade - being about .001" oversized seems to work well.

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Lately, I've gotten lazy and I've become a plated bullet fan as you can push a soft lead plated bullet harder than a soft lead non plated without leading issues. A lot of shooters like powder coated bullets for the same reasons.