Lower Charges for Heavier Bullets...Why?

Heavier bullet takes up more case. Accelerates slower so pressure is higher. more pressure. Max charge is dictated by pressure.

Eta. Yes thats simplified and I know there is more to it. Someone will be along with the rest

Heavier bullet gives more pressure.
Look up loads for any other cartridge and you will see this.

yea - I agree the heavier bullet has more mass to get going and will probably have more friction and contact with the barrel, and again get a slower start as a result, creating greater pressure out of the gate.

film495 said:

greater pressure out of the gate.

Click to expand...

All the way out of the barrel. And smokeless powder burns faster the more its pressurized so you get even more pressure

crestoncowboy said:

All the way out of the barrel. And smokeless powder burns faster the more its pressurized so you get even more pressure

Click to expand...

I'm relatively new to reloading, but just wanted to comment. It is these threads that remind me of why poeple should stick to only the listed loads, because the variables at play are non linear, and non intuitive, certainly until you've been doing it a heck fo a lot longer than I have.

Ironwill1406 said:

Why the lesser powder loads with heavier bullets? I would think you'd need more powder to push a heavier bullet just as fast or a bit slower...not less.

Click to expand...

Published load data are tested to same SAAMI max chamber pressures and powder charges are increased until same max chamber pressure is reached.

And here's physics formula:

So to maintain same/similar SAAMI max chamber pressures, if you increase mass, you must decrease powder charge (which causes bullet to accelerate).

Ironwill1406 said:

Xtreme 115gr 9mm bullets (copper plated) ... CFE Pistol

... also have Xtreme 9mm 124gr and 147gr ... Hogdon does not have Xtreme or even lead specifically at these weights

Click to expand...

Really? http://www.hodgdonreloading.com/data/pistol

• 9mm 115 gr Lead RN CFE Pistol COL 1.100" Start 4.9 gr (1,124 fps) 28,800 PSI - Max 5.4 gr (1,209 fps) 33,800 PSI
  
  
• 9mm 125 gr Lead CN CFE Pistol COL 1.125" Start 4.4 gr (1,041 fps) 27,200 PSI Max 5.0 gr (1,156 fps) 33,000 PSI

Here are comparison load data for TMJ which is Speer trademark thick plated RN.

• 9mm 115 gr TMJ RN CFE Pistol OAL 1.135" Start 5.1 gr (1094 fps) - Max 5.7 gr (1188 fps) - https://reloading-data.speer-ammo.c...m_caliber_355-366_dia/9mm_Luger__115_rev1.pdf

• 9mm 124 gr TMJ RN CFE Pistol OAL 1.135" Start 4.7 gr (1024 fps) - Max 5.3 gr (1127 fps) - https://reloading-data.speer-ammo.c...m_caliber_355-366_dia/9mm_Luger__124_rev1.pdf

• 9mm 147 gr TMJ RN CFE Pistol OAL 1.130" Start 4.1 gr (897 fps) - Max 4.6 gr (980 fps) - https://reloading-data.speer-ammo.c...m_caliber_355-366_dia/9mm_Luger__147_rev1.pdf

film495 said:

because the variables at play are non linear,

Click to expand...

Very non linear. Bullet setback from 1.160 to 1.130 (random numbers) may only increase pressure 2%. Then from 1.130 to 1.125 may make it spike. Some powders aren't as sensitive to setback either. Black powder doesn't burn much faster when pressurized. Some powders produce considerably more pressure when the temp is hot outside compared to when its cold. Some are pretty much insensitive to Temps.

Supposedly reducing the charge of win. 296 or H110 actually increases the pressure even using the same bullet and no other variables. Thats clearly stated in most every loading manual. Why? I do not know. Thats very much counter intuitive

Just restating what others have said in different terms but part of it is case capacity. Heavier bullets in the same caliber are longer. There isn't room for as much powder with heavier bullets.

And even if there is room there is pressure. Given the same powder charges behind a heavier bullet you'll see greater pressure. Possibly too much pressure for the gun to stay together. The power generated by smokeless powder isn't constant. The more resistance the more pressure it generates. Heavier bullets create more resistance and even the same powder charge will have more pressure.

If you want more speed with heavier bullets you need more case capacity to do it safely. That is why they make 357 Sig to get more speed than 9mm.

jmr40 said:

Just restating what others have said in different terms but part of it is case capacity. Heavier bullets in the same caliber are longer. There isn't room for as much powder with heavier bullets.

Click to expand...

You don't understand ammo. Charge weights are adjusted for different bullet weights for what is required to not exceed safe operating pressure. For example, in many pistol rounds there is a tendency for some powder charges to have a greater % case fill with light bullets, and for them to have less % case fill with heavy bullets. This means you end up with more empty case space with heavier bullets.

jmr40 said:

If you want more speed with heavier bullets you need more case capacity to do it safely. That is why they make 357 Sig to get more speed than 9mm.

Click to expand...

Not sure what you're trying to say here. Are you saying more speed with a heavy bullet than a light bullet, or with the same bullet weight?

fxvr5 said:

Charge weights are adjusted for different bullet weights for what is required to not exceed safe operating pressure.

Click to expand...

Yep. For example in 44 magnum you can have a full case of 296 at 24.5 grains . Or 1/3 case full of unique (10 grain) and both be at the same pressure with the same c.o.l. and bullet.

In rifles many accurate loads are full to the top of the case almost. The bullet actually compresses the powder when you seat it.

To the OP though. Case capacity is part of it some but only a small factor. Take a safe .40 round. Even a factory round. Seat the bullet deeper and it can be way over safe pressure. (Which is why you shouldn't rechamber the same round over and over. Bullet setback) More of it is because the heavier bullet will be harder to accelerate and slower to do so. which puts more pressure on the powder charge. The higher the pressure on smokeless the faster it burns. The pressure won't stop building or drop until the bullet exits the barrel.

A heavier bullet will 99.9 percent of the time be longer and have more surface area contacting the barrel and causing friction/resistance but again thats a small factor. IME

crestoncowboy said:

The higher the pressure on smokeless the faster it burns. The pressure won't stop building or drop until the bullet exits the barrel.

Click to expand...

This is false. Peak pressure is reached after the bullet has moved a very short distance, in many handgun applications less than a half-inch. Google search for pressure curves for actual data.

By the time the bullet has exited gas pressure is a fraction of what it was at peak.

fxvr5 said:

Google search for pressure curves for actual data

Click to expand...

Was supposed to read AND drop. But ill leave it now, so as not to confuse anyone reading later.

But yeah thats correct. Most every reloading manual I have has very good info on it. Very detailed. I don't think many people actually read the manuals. Lol. My first one was a very boringly written manual (speer or Sierra I think) that came with an RCBS partner press kit many years ago. Had a ton of great info but wasn't very entertaining so most skipped it. The Lyman 49th is pretty highly praised for its informative section too.

ETA. SPEER 13th is the one that came with the RCBS partner back around the 90s

Just to be clear, this is a pressure curve from a handgun (38 Super) with 2 powders, HS-6 and 231. The pressure peaks quickly and drops rapidly as the bullet travels down the barrel.

Less powder because heaver bullets are longer. Longer because they are the same caliber and the same material, so length must change if you increase weight while dealing with the same general profile. Longer bullets must be loaded deeper into the same brass to achieve the same overall length, required for things like fitting in a magazine or in a chamber. A longer, heavier object does not increase available space for powder burn. Hence you can't load the same amount of powder and get the same peak pressure. You have to load less. That means less chemically stored energy, less gas behind the bullet from deflagration, more friction with the longer bearing surface, so lower velocities. If you have less room and then load less powder, you have less has expansion, less ejecta, less pressure on the bullet when it exits the barrel due to quicker drops in pressure as the volume of the barrel increases behind the bullet as it moves forward.

You most certainly can push a heavier bullet faster with more powder, but not with the same cartridge if you are talking max charges with the same bullet construction and the same powder.