DO NOT load below any starting load you find for a 125 JHP bullet. Jacketed bullets have a much higher coefficient of friction than lead bullets, and trying to load jacketed bullets at target-load levels can result in a bullet stuck in the bore.
Let's do some more myth busting today.
The "jacketed bullets have a much higher coefficient of friction than lead bullets" comment isn't actually true, or at least it's a bit more complicated than just jacketed versus cast.
Jacketed bullets in general use a softer lead core to get better expansion, and because the jacket protects the base and the bearing surface of the bullet from gas cutting. Consequently they can use a softer lead core.
Way too many cast bullet shooters think you need a harder bullet for faster velocities and that's not the case, or at least not the full story.
A cast bullet needs to be able to obturate to seal the bore and needs to do it quickly to avoid gasses flowing past it in the throat and causing gas cutting, with that cut lead being deposited further down the bore.
The "harder bullets are needed for faster velocities" folks end up using a bullet that is too hard to obturate quickly with the available chamber pressure and then get leading. So of course the use an even harder alloy and make the problem worse. They end up using a gas check to manage the gas cutting, but still use an alloy that is too hard to perform well at the target. If you are just shooting paper or plates it's no big deal. However, a bullet that doesn't expand in a game animal, or worse shatters and under penetrates isn't a good thing.
A lead alloy is too soft only when the pressure behind it causes it to skid in the rifling and that can be complicated by a long throat or forcing cone where the bullet picks up more speed before it engages the rifling.
The fit of the bullet to the throat is also critical. If a bullet is around .001" under size for the throat, it will bump up to full diameter pretty easily and you can actually use a harder alloy without getting gas cutting. However if the bullet is under sized by .003 or so, you need a much softer alloy to get the necessary obturation. But it still must be hard enough to not skid when it enters the rifling and leave lead in the beginning of the rifling.
Some cartridges are less cast bullet friendly than others. For example, if you've got a large throat that is significantly above bore diameter, like the .375 Win, where the chamber and throat are oversized (to ensure a .380" .38-55 bullet can be cleanly released) but with a .375" bore, you have a potential pressure issue.
The shooter might choose a a bullet cast very hard for the 52,000 psi pressure of the cartridge and the fast 1-12" rifling (compared to 1-15" or 1-18" for the .38-55 parent cartridge) and sized around .379" for the large throat. That large and hard combination will cause excessive pressure when that hard bullet has to be sized from .379" to .375" in the throat to fit the bore, even though the .375 Win has a long tapered throat for that purpose.
A .375" jacketed bullet works fine in the large throat as it isn't affected by gas cutting. Similarly a modern .3775" diameter .38-55 jacketed bullet does fine in the .375 Win chamber as it's again not impacted by gas cutting and the bullet has a soft core that is easily sized to .375" in the tapered throat.
But, the .375 Win's combination of large throat, small bore, high pressure, fast rifling twist, and 2200 fps velocities with a 200 gr bullet and 2000 fps with a 250 gr bullet make it a serious challenge for safe and accurate cast bullet shooting. I left out "fast" here because something has to give and it has to be pressure and velocity if you want accuracy and minimal leading with a cast bullet. That's why I ended up going with a .38-55 in the end. It's much better suited to cast bullets in terms of pressure rifling twist and throat versus bore diameter.
Another example of a difficult nonjacketed lead bullet load is the FBI's old 158 gr LSWCHP .38 +P load. The bullet had to be soft enough to expand at what were still comparatively slow velocities, but hard enough to not cause leading issues that might affect accuracy over a 50 round course of fire. There's a very narrow sweet spot, and while Winchester and Remington found it fairly quickly, Federal struggled with it.
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You may have noticed that at no point have I talked about coefficient of friction. If ballisticians talk about it at all, the teen to use terms like "stickiness" and that refers to the friction of the bullet under pressure due to the combination of jacket alloy, core hardness and bearing surface. It's not all that relevant to a jacketed bullet versus cost bullet comparison where cast bullet peak pressure issues occur when a "hard cast" bullet has to size to bore diameter.
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A large part of the problem is that the definition of "hard cast" has changed over the years with many cast bullet shooters just referring to all cast bullets as "hard cast", with a few of those maybe thinking that cast bullets with BHNs of 10 or less being reserved for black powder firearms and the rest "hard cast" for smokeless powder use.
The sad fact is most of the cast bullet shooters have been influenced by commercial cast bullets that are often marketed along the lines of "hard is good, and harder is even better". Unfortunately that's not the case for all the reasons outlined above.
Swaged lead bullets are generally "soft" in the 8-10 BHN range as are bullets cast from alloys like 1:40 and 1:20 lead to tin ratio alloys.
Old school clip on wheel weights that were slow cooled were about 12 BHN and Lyman No. 2 alloy has a BHN of 15. Those alloys used to be considered "hard cast".
I use 1:40 (BHN 8) alloy in my .45-70 and .38-55 rifles up to about 1500 fps and use 1:20 (BHN 10) alloy up to about 1800 fps. These are plain based bullets with a poly fiber wad at the higher velocities.
The hardest alloy I normally use is Lyman No. 2 and that's normally on .357 magnum pistol and rifle loads.
Unfortunately, the "harder is better" crowd will be using those Lyman No. 2 and slow cooled WW alloys in "soft cast" applications and water quenched WW (BHN 18), Linotype (19 BHN) or oven treated WW (30-32 BHN) for hard cast bullet applications.
All they are usually getting with the overly hard alloys is excessive pressure and increase leading.
