Less powder with lighter bullet 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.

 

[ArchAngelCD]

It's not just the powder charge that raises pressure. Bullet type, seating depth and how close to the rifling all contribute to pressure.

 

[sparkyv]

It's not just the powder charge that raises pressure. Bullet type, seating depth and how close to the rifling all contribute to pressure.

Yup, and amount of crimp, brass volumetric capacity, temperature, to add a few more considerations.

 

[Steve C]

Apparently H335 is a volume sensitive powder that requires a heavier bullet to burn properly. The Hodgdon data shows a lighter 102 gr bullet load in the 7.62x31 using less powder producing a higher pressure than more powder using a 125 gr bullet.

 

[Skeptic13]

If you look at Hodgdon load data on their website for 7.62x39 and H335 they do show a heavier 150 gr SP bullet getting higher pressure with less powder than the 125 gr SP. One bullet is a Speer SP and the other is a Hornady SP but the construction of those bullets should be very similar. You might want to verify that the bullets are of similar construction in the Lee load data. I like you was under the impression that it usually takes less powder to load heavier bullets in the same caliber.

 

[Skeptic13]

Apparently H335 is a volume sensitive powder that requires a heavier bullet to burn properly. The Hodgdon data shows a lighter 102 gr bullet load in the 7.62x31 using less powder producing a higher pressure than more powder using a 125 gr bullet.

View attachment 870286

Yes but go one step further and look at the 150 gr bullet in the Hodgdon data. It shows a decrease in powder charge compared to the 125gr. The 108 gr is a Barnes Frangible bullet and I do not think it can be compared to either the 125 or 150 gr SP.

 

[Slamfire]

If you look at Hodgdon load data on their website for 7.62x39 and H335 they do show a heavier 150 gr SP bullet getting higher pressure with less powder than the 125 gr SP. One bullet is a Speer SP and the other is a Hornady SP but the construction of those bullets should be very similar. You might want to verify that the bullets are of similar construction in the Lee load data. I like you was under the impression that it usually takes less powder to load heavier bullets in the same caliber.

@ArchAngelCD really nailed it in his post.

Here is the thing, we all have our theories about how powders are supposed to work, and those theories are probably 90% wrong, maybe more, but since no one generally gets hurt, being right or wrong is not that important. But, powder companies have pressure gauges, not the old copper crusher stuff, but pressure gauges that provide digital readouts of the pressure curve, real time. And they see things that you will never see at home. So, if the factory gives you advice, based on measurements they have taken, follow their advice and ignore what you think you know.