Pistol Reloading Clarification

[Vintage BPCS]

Not sure where to place this thread, but I have a question.
Reloading for my Ruger GP 100-357 mag, I noticed in my various manuals that a powder charge for the same weights of bullets are different. Why isn't, as an example, 158 gr bullet loaded up the same, regardless of composition or shape etc. when using the same brand of powder?
Example using Lyman loading data a 158gn JHP using Acc #9 has a range of 13.4-14.9 then switch to cast LFN in 158gn with Acc#9 has a range of 12.4-13.7. Therefore can I use the loads data for a 140gn jacketed bullet to load for same weight cast bullet or not?
BPCS

[JakeBlanton]

Even when you are comparing jacketed to jacketed and lead to lead, you might find a difference since there is a difference in the surface area that is bearing upon the inner surface of the barrel. Given the same nose profile and weight, a hollowpoint is going to have more contact area with the barrel than a non-hollowpoint and thus more resistance to it sliding down the barrel. Of course, the depth of the hollow area can make a difference also.

Published reloading data is not necessarily just related to pressure. Sometimes it is related to group size. If a company sees the group size opening up, they might not continue testing heavier powder loads even if the pressure is not getting close to the maximum allowed.

According to the Lee book, a 140 gr lead bullet with Accurate #9 should be 11.3 gr (1204 fps) to 12.5 gr (1338 fps) with a min OAL of 1.590" and a max pressure of 33,700 psi.

[Vintage BPCS]

I have noticed that as the bullet weight increases the powder charge tends to decrease too.
So can I assume that if friction/bearing surface is the factor behind powder charges, that for cast bullets with more grease grooves and less bearing surface that I could then use the jacketed bullet data if I stayed at the lower side the load table charge. You explained why the difference in my very old Speer manual verses the new Lyman Cast manual.
BPCS

[grumman581]

I have noticed that as the bullet weight increases the powder charge tends to decrease too.
So can I assume that if friction/bearing surface is the factor behind powder charges, that for cast bullets with more grease grooves and less bearing surface that I could then use the jacketed bullet data if I stayed at the lower side the load table charge. You explained why the difference in my very old Speer manual verses the new Lyman Cast manual.
BPCS

It's not just bearing surface. There is the inertia of a bullet that affects its acceleration down the barrel. As such, you need less powder so that you do not exceed the specified pressure for your caliber.

Here's a post that I made on the "10mm/.40 (http://castboolits.gunloads.com/showthread.php?215832-10mm-40)" topic that might be of help:

http://castboolits.gunloads.com/showthread.php?215832-10mm-40&p=2429823&viewfull=1#post2429823

[Vintage BPCS]

If my physics is correct, then bearing surface combined with bullet weight must be overcome by inertia. So for a given charge of powder the pressures may vary as explained in the thread that was referenced.
Thus if I were to stay at the lower powder volume for a given jacketed bullet weight when loading for cast bullets of the same weight, I shouldn't get into much trouble if I work up the loads over a chronograph and don't exceed the published velocities for those applied components.
BPCS

[theperfessor]

There is also OAL and how far the bullet sticks into the case. A cartridge with the same weight powder and bullet that has different internal capacity will of course have different pressures. Why else would we worry about bullets telescoping into the case in an auto loader? Try loading a 140-150gr .38 wadcutter flush with the case mouth, and then only seat it to the crimp groove, the top lube groove, and then middle crimp groove. Go shoot ten or twelve of each with a powder charge that is safe with the flush seated bullet and pay attention to the recoil and report.

Now look at a few different 158 SWCs and measure from the crimp groove to the base.

Also, even the same company using the same testing methods, etc. won't come up with the exact same data every day. Close, but not identical. And testing methods between different companies and during different eras is also a factor. I try to use the most up to date info I can, unless it is for a powder that is no longer made or a bullet weight nobody lists.

[grumman581]

If my physics is correct, then bearing surface combined with bullet weight must be overcome by inertia.

In our discussion about ballistics, inertia is something that must be overcome, not something that overcomes. Inertia is basically the resistance to movement of an object. It's also related of mass, not necessarily weight, so it would be something that you would need to take into account even in a weightless environment.

The burning of the powder and the resultant gases result in a force being applied to the base of the projectile. The powder does not instantaneously convert itself into gases, but rather starts burning and producing gases. The bullet starts going down the barrel and the powder continues to burn, resulting in more gases trying to expel the projectile from the barrel. Only so much gas can be produced for a given amount of powder, so there is a maximum length of barrel for a particular powder that will result in a maximum velocity. It is also possible that you could load a round such that you are not producing enough gas pressure to overcome the resistance (frictional and deformation caused by the rifling) of the bullet traveling down the barrel.

Are you trying to create a high velocity round or a very low velocity round? There's plenty of data out there to give you a starting point and you can either go up or down from that point, depending upon what you are trying to do.

[MtGun44]

Different boolits/jbullets have different bearing lengths, different materials (friction coefficient) and different hardness (work level to engrave rifling)
defferent inertia (heavier so takes more work to accelerate - think semi vs Honda civic - how much HP req'd?) AND different amount inside the
case (changes combustion chamber volume, this is a BIG DEAL).

Very significant differences with different components. Similarly dimensioned and similarly manufactured jbullets should be fairly similar IF SEATED
to same base location. Boolits generally produce lower pressures than same weight jbullets, but not always.

Bill

[Vintage BPCS]

OK I'm getting the idea. I will start at the lower charge level for similar apparent bullets and reload to spec. Test over the chronograph for the best grouping. Just basic reloading as I have been doing all along for all the calibers I load for.
BPCS