Someone please explain to me why!

[bowman1962]

I know that there is a simple explanation for it but why is the powder charge heavier for lighter projectiles?

Example;

44 Mag.
180 gr. Hornady XTP ( Maximum load w/ IMR 4227 is 29.0 grains )
240 gr. Hornady XTP ( Maximum load w/ IMR 4227 is 24.0 grains )

I just can't get my head wrapped around why it takes more for less




bowman1962

 

[JustSomeGuyinCA]

A 180gr bullet is lighter than a 240gr bullet. A 180gr bullet is also shorter than a 240gr bullet. A 180gr bullet seated in a case to the same OAL as a 240gr bullet = more space in the case. More space in the case = lower pressure for same powder charge. To get higher pressure and velocity, add more powder.

 

[JRWhit]

take the charge for a 180 gn bullet. A 200 gn bullet will be more resistant to move than the 180gn. There fore it would produce higher pressures behind the 200 gn than the 180 gn. The correct charge behind a 180 gn bullet would result in over pressuring behind a heavier 200 gn bullet surpassing never exceed pressures.

Basically same thing some guy in CA said just in the other direction.

 

[Magnum Shooter]

A heavier bullet is harder to get moving than a lighter one. In order to keep pressures from reaching a disastrous level, less powder is used.

 

[ljnowell]

The heavier bullet has more resistance to moving. That resistance to moving is what allows pressure to build. The lighter bullet has less resistance, therefore it will take more powder to build a similar pressure under the bullet.

 

[GeneCC]

I would GUESS that it has to do with how powder works. I don't have the equations of combustion handy but you can figure it out logically.

People tend to view a bullet going down a bore like a spitball through a straw. You blow on it with more or less constant pressure, accelerating the bullet down the bore.

The reality with combustible powders is different.

The rate of combustion of a powder is proportional to the pressure of gas around the grain. The higher the pressure of gas the faster that it burns or deflagrates.

If you just pour smokeless powder onto a flat surface and light it it fizzles slowly. Atmospheric pressure isn't much.

Put it inside of a pressure vessel, ignite it and the pressure rises to a point where the vessel ruptures or the powder is consumed. Whichever comes first.

The higher the pressure the faster the combustion.

So you have this closed container, where the powder burns at some rate because of pressure against it.

Pressure and volume are inversely proportional. Increase the volume of a gas and its pressure drops.

Think of the cartridge and barrel as a long tube. The powder is ignited by a primer. The pressure rises to the point that the bullet is pushed out of the cartridge into the barrel bore. The butt end of the bullet seals the tube at one end. The other end is sealed off by the cartridge.

Inside of this pressure vessel the powder burns, dependent upon the pressure.

A heavier bullet is "more resistant" to motion. Inertia. So the "seal" in the pressure vessel will move more slowly. Slower change in volume means that the peak pressure goes higher than with a lighter bullet.

Think of powder as the amount of energy you can safely put into the system. You have to go lighter with heavier bullets to keep from blowing it all up.

With a lighter bullet you get a faster change of volume. The lighter bullet has less resistance. So you can push the bullet with more energy.

The rate of change of the volume with a lighter bullet will be faster, all factors being equal (same powder, primer, case and diameter of bullet). This faster change in volume "burns" the powder slower because the rate of change of the volume is faster.


This is a lot easier to explain with some equations.... or some graphs.



The green is the internal pressure of the bore (it's a mythical average because the pressure will vary all through the combustion zone), the blue represents the velocity of the bullet in the bore, and the red its position.


If you use a heavier bullet that green curve will "rise" because the powder has to work harder and thus the gas pressure will rise to drive combustion to a higher rate. If you go too high the pressure vessel ruptures.

So those values are not "stuff" but more like a speed limit. It's more "what can I get away with before I rupture this thing?"

 

[bowman1962]

Think of powder as the amount of energy you can safely put into the system. You have to go lighter with heavier bullets to keep from blowing it all up.

With a lighter bullet you get a faster change of volume. The lighter bullet has less resistance. So you can push the bullet with more energy.

The rate of change of the volume with a lighter bullet will be faster, all factors being equal (same powder, primer, case and diameter of bullet). This faster change in volume "burns" the powder slower because the rate of change of the volume is faster.

Now that makes sense!

Thank you all for the enlightenment.

 

[jim243]

Let's see if I can explain this right. It is not so much the weight of the object, but the space with in that the propellant burns.

If you took a 1 cubic foot square and burned lets say 25 grains of smokless gunpowder to push a 1 lb object. You would produce X amount of pressure (foot pounds) on the sides of that cube to push that object.

Let's take that cube and change the internal dimensions to 1 foot by 1 foot by 1.5 feet (it is a cube after all). That same object that you are trying to push will require MORE gun powder to push that same one pound since the gas is expanding in a larger space and producing fewer foot pounds of pressure.

Heavier bullet are generally longer than lighter bullets (there are exceptions) and take up more space in that cube. Taking up more space reduces the total volume that the gun powder will have to burn in and produce higher pressures. To adjust for those higher pressures you reduce the amount of gun powder you use with heavier bullets to bring the pressure level back down to a sale level.

So with heavier bullets you use less gun powder and with lighter bullets you use more gun powder to generate the same PRESSURE level in both.

I hope that helps.
Jim

Heavier bullets also have more drag (surface area) in a barrel and will up the pressure to unsafe levels without reducing the amount of powder.

 

[Trent]

Yup, it's all about case capacity vs. burn rate vs. projectile weight.

This is why (as I recently discovered) old-timers will seat ~150gr wadcutters REAL deep in to 357 cases, to reduce the case capacity, so lighter target loads will shoot more consistently with light powder charges. Seat the bullet longer and the powder doesn't burn as steady.

I've been experimenting with this along similar lines recently with rifle cartridges and various powders in 7.62, 8mm, etc. I'm re-treading old work done by pioneers and finding they knew their business; it's part of the reason why each caliber tends to have a "preferred" powder, when a dozen or more different powders might work out fine.

Sierra and others allude to this with "accuracy" load listings, you'll find the accuracy load powders tend to be the powders that are the right "bulk" for that cartridge at the rated pressure of the cartridge. Powders that mate well to the given capacity and pressure of the load tend to be the most consistent when burning, vs. having powder loose, or overly compressed.

 

[45lcshooter]

Heavier bullet, means a longer bullet, which means not a lot of room for more powder. less weight bullet needs more powder. A bullet doesn't come out the end of the barrel because of fire, it comes out because of the pressure from the fire burning the powder, thus pushing the bullet out the barrel.

It takes more pressure to push a light bullet. More volume inside the caseing to create pressure.

 

[jerkface11]

If you use the same powder charge with the a different weight bullet the velocity will be the same. However as pointed out above you can't use as much with a heavier bullet. Also with some powders you have to use more with lighter bullets or it won't ignite properly.

 

[Walkalong]

The lighter, shorter, bullet is easier to move. Pressure builds up a hair slower so you can use more powder.

 

[345 DeSoto]

You learn something new every day...

 

[bluetopper]

The point in it all..........is to keep chamber pressure below a certain threshold.

Pressures will build up more moving a heavier bullet so you have to use less powder to keep the chamber pressure down.