Less powder with lighter bullets to prevent over pressure

[dgang]

While loading some 7.62x39mm with 125gr. FMJ using 30gr. of H335 I noticed Lee's load guide advising not to use that amount of powder with a lighter bullet in order to avoid overpressure. It seems contrary to my experience. I thought a heavier bullet of the same construction requires less powder to keep the pressure at safe levels.
Does anybody have an explanation for this contradiction?
Thanks in advance, dgang.

[Marco Califo]

...

[TX Nimrod]

Best to check the data before you badmouth someone else. The Lee manual is correct in that current Hodgdon data shows a smaller charge of H335 behind a 108-grain bullet compared to a 125-grain bullet - 28.8 versus 31.5. Odd yes, but fact.


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[mikld]

Does anybody have an explanation for this contradiction?

??

Personally, my Lee manual is the last one I check for data (not a Lee Hater). I have found the data lacking and sorta "mish-mash" in that it seems old and inconsistent. I liked the "front half", but very rarely use the data section...

[FrankenMauser]

Does anybody have an explanation for this contradiction?

Absolutely: Lee's data sucks.

Lee's data is aggregated. They collect everyone else's hard work, mix it up in a big bowl, and pour it out onto a page in an amalgamated slurry of nonsensical garbage that lacks the fine print for the actual data creator's original test conditions.

[Ben Dover]

Check other manuals!

Contrary to what some folks believe, reloading manuals are NOT Holy Scripture nor Divinely Inspired,

They are often wrong. I like to check and compare at least three different "big name" manuals before trusting their data.

[T. O'Heir]

"...Lee's load guide..." Lee does no testing, of any kind, themselves. Most of their data comes directly from Hodgdon or the powder maker. They do not just help themselves though. That'd violate copyright laws. They buy it and publish it as their own. Which is bad enough.
30 grains of H335 is the Start load for a jacketed 125. 28.8 is the MAX load for a 108. You'd never use the same amount of powder for 2 different bullets. Do not mix load data.
The odd part is that Hodgdon's site(that's kind of questionable anyway) shows the 108 Max pressure as being higher than the 125 Max load. (The SAAMI max pressure for a 7.62 x 39 is 45,000 PSI. There is no converting CUP to PSI using mathematics.) Likely because the 108 is a Barnes Frangible, powdered-metal, copper-tin, core inside a guilding metal jacket bullet. Those are not the same as a lead cored bullet.

[Unclenick]

Dgang,

Mr. O'Heir is correct. The lighter bullet is a Barnes Frangible. They are likely made of a sintered copper powder that is not only less dense material than the Sierra SP but is even less dense than a Barnes copper solid. As a result, the bullet is rather long for its weight. In the little 7.62×39 case, the extra bullet length takes up a lot of powder space, increasing the initial confinement the powder experiences. That is why the charge has to be smaller, as it is not only for H335 but for every other powder Hodgdon lists for both bullets.

[dgang]

T. O'Hair, Unclenick, The Lee data which was included with the die set has both the Barnes X solid 108 gr. bullet and an entry for 108 gr. Jacketed Bullet. I was referring to the jacketed bullet as I know that all copper Barnes X solid bullets are not comparable to jacketed bullets. Looking at other data in the Lee "Book of Modern Reloading" shows similar CUP pressures for both( about 40,000) but the 108 is going much slower, around 2100 fps as compared to 2400 fps for the 125gr. jacketed bullet. Still puzzled why two bullets of jacketed construction would have the same pressure when one is heavier and faster. Lee may have transposed things a bit. Thanks for your input.

[Unclenick]

Again, the Lee data is mostly culled from disparate sources that could well be from different guns using test cartridges loaded into different cases with different primers and, of course, different lots of powder. In the case of the Hodgdon data, at least, you are seeing the powder and component mix kept constant. The guns may be different if one is a copper crusher and the other a piezo transducer.

The Hodgdon data for the Barnes bullet is for a frangible bullet, not a copper solid like the X bullets, so it will be still less dense and even less comparable to the cup-and-core bullet.