Something I've Never Really Understood: Relationship between bullet weight & powder load

[orpington]

I have never understood this relationship and suspect it has to do something with bullet profile/aerodynamics and coefficient of friction and percent of bullet contact with barrel???

Let's use .44 Special as an example, 4 different bullets, two with identical grains, let's use a powder choice of Unique consistently.

Lyman 429667, 240 grain bullet. Starting load is 5.8 grains. Maximum load is 6.5 grains.

Lyman 429383, 245 grain bullet, Starting load is 5.2 grains. Maximum load is 6.5 grains.
(One would think that more grains needed to move a greater weight bullet, but not the case???)

Lyman 429421, 245 grain bullet, identical grain/weight as the previous bullet, starting grains is 6.0 and maximum load is 6.9 grains. Appears to be no relationship, inferred or otherwise???

Lyman 429244, 255 grain bullet, starting grains 5.7. Maximum load 6.6 grains.

I can figure out no relationship whatsoever.

So how then is starting and maximum load determined? Is it simply experimentation with desired minimum velocity for starting load and maximum C U P permissible as maximum load?

And thus how would maximum pressure be determined? Apparently with Lyman 429421 13,300 is okay, but with 429667 only 12,400 is okay. Why?

This would be nice to know as I created a starting load of 5.8 grains Unique behind Lyman bullet 429667, and the maximum load is 6.5 grains. I loaded several before noting the scale was at 6.5. Since I NEVER exceed published loads, these rounds were scrapped. Not sure why a probable pressure exceeding 13000 C U P in that load could not have been acceptable, since a similar C U P is acceptable in other examples. Easier to have fired those rounds instead of disassembling them.

 

[Jack Ryan]

The heavier the bullet, the harder it is to move. So the higher the pressure it is going to take to move it. What ever happens after it leaves the barrel has no affect on the pressure created inside the barrel so aerodynamics has no affect other than if the shape causes the bullet profile to have more contact with the barrel increasing friction.

 

[TxRoadDog]

Are you getting your information from different loading manuals? If that's the case, you are introducing many variables. Sometimes heavier weight bullets have smaller chargers because they take up usable case capacity due to longer length. It just depends.

 

[35 Whelen]

If you compare the length of the bullets from the crimp groove to the base, my bet is that is where you'll see a difference. All things the same, the greater the space a bullet takes up in the case, the higher the pressure will be.



35W

 

[orpington]

All data was from the same Lyman reloading manual.

 

[orpington]

Also, for example, OAL for 429667 should be 1.500". Ones I loaded today using starting load of IMR 4227 at 12.7 grains (max would be 14.2 grains) were at 1.506" except first round loaded was at 1.469" and was set too deeply. How much would this affect pressure at .031" too deep?

 

[fxvr5]

First, whoever collected data for the manual did not follow the exact same criteria with respect to maximum pressure limit, as you have already observed. That muddies the comparison.

When you do use the same value for the maximum pressure, and the bullets are constructed the same, such as cast from the same alloy, other factors can decide when the maximum pressure is achieved. Those are: bullet weight, bullet length, bullet shape (which determines length) and the overall length (seating depth). When you consider all these factors, and how they affect the available space in the case, it all makes perfect sense. Heavier bullets require less powder, and will travel slower, than light bullets. Seating the same weight bullets deeper in the case increases pressure, and so on.

Starting charges can be somewhat arbitrary, such as the max load minus 10%, though for some powders it might be important.

 

[Riomouse911]

There may be other factors as well.

The amount of bearing surface on the bullet touching the bore will increase or decrease pressure depending on bullet styles, so a lighter bullet with a greater bearing surface may create higher pressures with a lower powder charge due to friction.

The hardness of the bullet alloy may also be a factor for the velocity the manual is looking for... especially if it’s listing a bullet mold but not the alloy used. If it’s really soft it may lead badly at a higher velocity, so the manual writer found a lower velocity /pressure worked better for their purposes. This may make the case for loads published with a lower charge weight with lighter bullets as well.

All reloading manuals are just a guide that give the “average gun” loads that should be safe. It’s the challenge of finding that “just right” combo of bullet style, accuracy, velocity and consistency that makes handloading fun!

Stay safe!

 

[Livin_Cincy]

A heavier bullet requires more energy to start moving and to move down the barrel.
So the pressure created by the powder is not relieved by the bullet leaving the barrel as quickly.
The amount of time that pressure is pushing the bullet to escape the barrel is the cause & effect.

The smaller amount of powder with a heavier bullet has more time to build pressure than.

 

[Walkalong]

When looking at the difference everything is the same except for the bullet weight (Gun, case, etc). Since we are limited by peak pressure the heavier bullet can only use a lesser amount of powder as it will peak faster trying to move the heavier bullet with the same speed powder in the same case/gun. More weight, more friction, harder to move, so pressure builds faster, and so we are limited to less powder or we have to change to a slower powder to get more velocity if that is what we need.