Just got some Bayou coated LSWC in 200 and 185 gr to try and looking at the load data notice the difference in powder charge between the two weights. With BE86 the max charge is 6.7 for the 200 gr and 7.5 for the 185.

Why does the lighter bullet take more powder that the heavier. That just seems backwards to me.

The heavier bullet is slower to get started and move through the barrel, therefore the pressure has more time to build up to max.

A lighter bullet moves out fast, with the volume in the barrel/chamber behind the bullet expanding rapidly as the pressure builds.

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"He was kinda funny lookin'"

That makes sense. Thanks. I can now sleep soundly without that question weighing on my mind.

Some things seem counter intuitive. There are some 'facts' about some bullet lubes and velocity that I can't justify though I've spent many a night contemplating physics, common sense and the fact that some things are counter intuitive and don't make sense. But I'd at least try to have an explanation that I can understand.

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"He was kinda funny lookin'"

Typically, the 2 major things that affect pressure are:

The amount of bullet in the case.
The weight of the bullet.

Back in the day bullseye powder and 148gr wc's had allot of issues (kabooms). So alliant put this out to try to educate reloaders on pressure levels vs seating depths (the amount of bullet in the case). They covered different seating depths along with double charges.



Fast forward, today the 9mm and 40s&w's are the bad boys on the block. There's allot of problems with bullet setback (the amount of bullet in the case) and pressure spikes/kaboom's. Powder mfg's started coming out with charts like these showing them using the same powder charge and pressures rise as the bullets are seated deeper.



A extremely common combo, the lee 120gr cast bullet/5.0gr unique/9mm case. As the oal/bullets seated deeper, the pressure rises for the same load.



2 bullets can have the same amount bullet seated in the case. The heavier bullet will have more resistance which ='s more pressure build-up for the same powder charge.

Good data, thanks.

What book did the pressure data come from? Would like to add that one to my reading collection.

As Forrest notes, weight often affects bearing length, so less powder for less equiv pressures, but the bullet will have less vel, more mass to move. So keep in mind, always safe to use heavy bullet data for a lighter bullet, but not the other way around.

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NRA Cert. Inst. IDPA CSO

Forrester is onto the correct answer, same thing happens with my 10mm 180 gr LRN and 205 SWC, with indenticial powder charges the 205gr will be 20-40 fps, faster, it also has 1/8 more bullet length inside the case, thus reducing the case volume and increasing pressure and delivers more velocity.

Its not just the compression of the powder but you are also pushing a bullet with longer bearing surface, greater friction to overcome, higher pressure, usually higher vel with the same powder charge.

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NRA Cert. Inst. IDPA CSO

Been looking for long time for precisely that type of info, charts and diagrams, ie matching pressures to specific load recipies. Where is this info found? I have no fancy gear, not even a chrono so my only pressure measurement is how it feels going off in my hand.!!(that almost doesn't sound right)
Have loaded 9mm, 380 and 32 acp towards max end of published recipies with no noticable signs of pressure excesses.
SAAMI gives max pressure with no directions on how to get there/avoid it.
Currently I'd like to work up loads for 32 acp with heavier projectiles, to 95 gr if possible.
Forest r, I'd like to read more. What is the source of your info please. Thanks

I did a little quick math on my 9mm bullets.
Xtreme plated 115gr at +/- .55"
Xtreme plated 124gr at +/- .59"
Speer TMJ FP 147gr at +/- .645"

My 9mm case was +/- .745" in oal
with a inside case height of +/- .59" volume space.
With all bullets set at a OAL of 1.14" .....
the bullets took up the volume area were as followed.

The little 115gr filled up about 27% of the case volume.
the medium 125gr filled up around 33.9% of the case
while the large 147gr took up a big 42.3% of the case !!

Now I understand why the load data is so low with the larger bullet. The 147 at a full 1.16" OAL is still seated the depth of three nickels. The little 9mm can get crowded in a hurry.
Stay safe.

As I've posted many times, interior ballistics is not only more complicated than most understand, it's more complicated that anyone without a working knowledge of advanced calculus CAN understand, involving complex set of second order differential equations. That means the relationships are NOT linear, and "common sense" does not work.
That's why we have and (hopefully) use the empirical load tables for reloading cartridges.

And as for looking for "pressure signs" in straight walled pistol cases, the first observable pressure sign may well be the top strap hitting the ceiling.
You can buy strain gage equipment and put it on your barrels, but strain gages on non-standard barrel profiles are notoriously hard to calibrate. Ask any engineer that took the lab course involving work with strain gages.

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Science plus Art