Slow powders vs. fast powders

[ready4shtf]

So I just got started in reloading 308, and so far all my loads have been with recommended powder and factory bullets and standard loads. I knoticed in my reloading manual, out of 50 different powder types the Varget powder I'm using is the 2nd slowest burning powder out there, with the 748 being the slowest.

Why do some cartridges use faster or slower burning powder?

I guess I dont understand why burn speed matters. Google isnt helping either.

 

[Fatelvis]

The optimum powder burn speed for a given cartridge, is determined by a whole bunch of factors. The main factor that determines what general speed is best, is the bore size to case volume ratio. Another is, generally, heavier bullets do better with slower burning powders, that would be suitable for the given cartridge.

 

[ready4shtf]

Ah, so a 45-70 and a 7mm Mag would definately not use the same powder... I think I get it.

 

[Sunray]

"...not use the same powder..." They can. Both can use IMR3031 and IMR4320. The 4320 isn't suitable for a Trapdoor Springfield though.
Powder burn rates determine how fast the gasses are created. Hence the pressures obtained. Fast burning powders create higher pressures than slower when used in equal amounts. Everything involved in loading has something to do with how pressures are created and contained. There should be a 'reference section' in your manual that explains it much better.

 

[aakoksal]

Hi,

To make it more simply, think of gun powders used for 9mm round and a 7mm magnum round.

Lets say a 9mm handgun has a barrel around 5". You want to make sure that you gun powder will completely burn within that 5" (+ your chamber). So you will need a fast burning gun powder.

Now lets think of a 7mm round fired from a 24" barrel. You still want all your gun powder to burn ideally just before bullet leaves barrel. If you use a very fast burning gun powder, all your gun powder will be burned, let's say, within 10". This will create an extremely high pressure which may even damage your rifle. If you keep your pressure within specified limits by using less amount of poweder, than your velocity will drop down...

On the other hand, if you use a slow burning powder, you will get high speeds with less pressure.

 

[Bottom Gun]

Faster powders are easier on barrels than slow burning powders.

 

[rcmodel]

Another way to look at it is:
Try to imagine smacking a socker ball and a bowling ball with your fist.

Both are about the same size, but very different weights. (Light bullet / Heavy bullet of the same caliber)

You will have pretty good luck hitting the socker ball really hard (fast powder).

But giving the much heavier bowling ball a little more time to accelerate up to maximum speed (slow powder) might be a wiser choice.

(If you don't want to break your hand.)


rcmodel

 

[brickeyee]

All the powder, fast or slow is completely burned within a few inches of bullet travel.
Very fast powders may be completely done before the bullet heel clears the case.
Slower rifle powders might make it for a few inches of bullet travel.

Peak pressure is what blows up guns, while average pressure (area under the time-pressure curve) is what creates high velocity.
'Fast' powders reach the peak sooner than slow powders.
The whole things becomes more complicated since burning rate is a function of pressure. Higher pressures increase the burning rate.
Neco is supposed to have a demo of Quickload available.
This is an internal ballistic program that uses actual equations to describe how the powder burns and allows plots of pressure vs. time or pressure vs. bullet position. When used carefully it can be very accurate.