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When case volume decrease and bullet weight increase how much does powder weight decrease

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I'm just trying to get some understanding of load theory. From my understanding, when going to heavier bullets in the same caliber, the factor preventing obtaining the same velocity is that the volume in the case will decreases because the bullet is presumably longer, so you can't use the same amount of powder or you will exceed the pressure limit whatever it might be.

But let's say we have two bullets, both copper plated with the same dimensions, one with a lead core weighing 180 gr total, the other one with lead+ core, weighing 200 gr. That's a 11% weight increase.

How would that compare to an 11% case volume decrease for two bullets with the same weight and obviously different dimensions?
 
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Not sure if it can be simplified as much as what you're saying. There's more to pressure increases than just weight. Bearing surface and composition of the jacket/bullet come into play as well as the amount of space left in the case.

Heavier Bullets will create more pressure to move at the same speed as the lighter ones.

More bearing surface creates more friction and resistance so that can also create more pressure.

Shoving the bullet deeper into the case will increase pressure to some point. Seating the bullet close to or jammed into the lands can increase pressure.

All of these things come into play when pressure and thus speed is concerned.

A heavier bullet will be more resistant to movement, but it's not necessarily going to go slower because you've lost case volume.

Many times the bullet can be set out further to make up for lost case volume as long as it's there's still enough clearance between the ogive of the bullet and the lands, but that doesn't mean you can fill the case up with more powder without going over pressure.
 

po18guy

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Fairly simple physics - those with degrees can explain better. However, a given pressure will accelerate a given projectile to a given velocity. A ton of assumptions and variables here, but go with a lighter bullet and it will be accelerated more briskly by the same pressure in the same length of barrel. That pressure (think: engine horsepower) simply has less work to do. With a heavier bullet, the pressure (or engine) must work harder (i.e. over a longer time) to accelerate the heavier bullet to the same veloicity as the lighter bullet.

And, according with those laws of physics, the heavier the bullet, the longer it takes to accelerate. Since barrel length is limited, the lighter bullet which accelerates more quickly has the velocity advantage. BUT: the heavier bullet will have the energy/momentum advantage.

The first few chapters of each loading manual are devoted to the relatively dry subject of internal ballisitics. Thus, they are rarely read and digested. However, a read of any one of those introductions will shed much light on your questions.
 
'Tis a great question, but I disinvite you to test theories. Testing is done at great peril.

Simple physics : force = mass x acceleration. That is what works in a vacuum. In a barrel, it's a lot more parameters to the equation:
  • you are deforming the bullet while pushing it down the barrel,
  • contact bearing surface of the bullet inside the barrel
  • twist rate of the rifling affecting the friction
  • changing equation of static vs. dynamic frictions
  • the burning rate of your powder
Those are just a few. As you travel deeper down the rabbit hole, barrel harmonics affect ultimate velocity. For example, you can increase a 1/2 grain in a 40gr powder charge (1.25%), but see a 70fps increase in muzzle velocity over the 2500 fps you were seeing (~3%).

This is also why there are different powder burn rates. The acceleration and force doesn't just happen in the chamber. The burning powder continues to accelerate the bullet down your barrel. If you're shooting the 338 Lapua cartridge, using 300gr projectiles, the same charge of RL26 vs N570 may give you a slower velocity in the RL26 plus a stuck bolt and case where the N570 gives you better velocity and controlled pressure.
 

po18guy

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Distilled to its essence, what the ballisticians have come up with, defined as maximum loads, is the best that can safely be done. I would suppose that many are the 'walking maimed' who have tried to reinvent the laws of physics.
 
OP
pinne65
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Thanks everyone for your input!
Sorry if wasn't clear enough. I wasn't looking to fill up any extra volume with more powder in order to push a heavier bullet at the same velocity as a lighter one.
What I'm wondering is, how loss of volume in the case compares to just a heavier bullet in regards to powder charge and pressure.
My thought experiment might also not have been complete, so here we go again:
Let's say we have two jacketed handgun bullets, (not sure why I wrote copper plated in the 1st example), with the exact outside physical properties. The only difference is that one weighs more than the other.
Then we have two bullets that weighs exactly the same, but one is longer than the first one. Other than that they have the same type of outside physical properties.
To me it seems that the longer and heavier bullet would demand a smaller powder charge than the one that's just heavier since it has both less case volume and a larger surface area while traveling down the barrel.

In short, is it worth chasing a shorter bullet with the same weight in order to either keep pressure down or possibly have a slightly higher powder charge than for a longer. (In the real world the shorter bullet would probably be less tapered rather than filled with heavier lead :)).

I started thinking about this because I decided to get some 200gr XTPs instead of the regular 180gr I usually shoot, and compared them to some copper plated 200gr flat points I have. L->R 180gr xtp, 200gr fp, 200 gr xtp
IMG_20200212_192732.jpg


Cheers!
 
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I believe this really does take some actually experimentation - with good pressure measuring devices a safe distance and the willingness to sacrifice a rifle or two.
Everything that may happen in a fire arm is not readily predictable, perhaps not even by Army artillery guys who may know a bit.
My background is as a metallurgist, experience here a few failure analyses of burst muzzle loaders and some knowledge of Remington shotguns.
 

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