Powder burn rate

[AK Eggy]

Yesterday, I posted a question asking if anyone had any load info for a short barrel .44 mag "Short barrel .44 mag load" and I ended up even more confused.

I was thinking the load for the same bullet between a 7 1/2" barrel and a 2 1/2" barrel would use different powders to optimize velocity. I'm trying to ensure good expansion w/ a hollow point. My line of thinking was the shorter the barrel, the faster the powder needed to burn and I guess that's not right.

I noticed in the current Speer manual that they have some short barrel load tables and found that the volume of the same powder for the same bullet is lower in the short barrel load tables. Also, obviously, the velocity is lower as well. I was assuming that was the case because once the bullet left the barrel, all the powder that had not yet burned was useless (except for a cool light show).

After yesterday's post and the info obtained from it, I think I see how it works and what I got out of it, I think, is that regardless of the powder used, a barrell of a certain length will only move a bullet of a certain weight a fixed velocity. Do I have it right? The fact that the barrel length is the parameter that would determine bullet velocity? When the powder is still burning as the bullet leaves the barrel, that is the max. velocity you'll achieve?

If that is the case, why the different burn rates for powder? IE, H110,Blue Dot, and Unique? WHat would be the advatage of using one over the other? What is the advantage of, or when do you look at the faster burning powders? When does the burn rate of the powder come into play? Also, How does the burn rate come into play?

On a side note, I see alot of .44 mag and the like loads, people prefer H110, and I do too, but mostly because of the ease of metering it in the powder throw. I tried some Blue Dot and came pretty close to matching the H110 velocities, but didn't care for it in the powder throw. What are the reasons most don't use Blue Dot in the .44 mag.?

I'm sure these are basic principles, but I'm still trying to wrap my mind around them.

Thanks for any help you can offer.

 

[rcmodel]

Fast powder reaches peak chamber pressure before maximum velocity is achieved.

Slow powder pushes on the bullet over a longer time frame before reaching Max allowable pressure for the caliber.

Think of it as trying to move a bowling ball.

You can hit it as hard as you can with your fist. (fast powder)
And break your fist without moving the bowling ball very fast.

Or you can give it a hard shove with the palm of your hand, and give it a more gradual acceleration without hurting yourself. (slow powder)

It is possible to move the bowling ball much faster by pushing it then by smacking it as hard as you can.

And yes, a longer barrel will always give higher velocity then a short barrel, with any powder at max allowable chamber pressure.

Conversely, slow powder will always give higher velocity then fast powder, in any barrel length with max allowable chamber pressure.

rc

 

[counterclockwise]

For a given cartridge, caliber, and bullet we select the powder to avoid exceeding the maximum chamber pressure, and have enough reserve gas volume and pressure after peak burn to propel the projectile out of the barrel as fast as practical. That's about it.

Too fast a burn and the case fails or worse. Too slow a powder and the compressed gas volume has less stored energy, resulting in a lower velocity.

Peak burn / peak pressure typically occurs only few inches down the barrel.

 

[Galil5.56]

found that the volume of the same powder for the same bullet is lower in the short barrel load tables.

You mean weight right? If given the same volume, a propellants weight will vary by virtue of its physical shape, technique, climate, density, etc... Also, the actual amount of energy that can be transferred over the time/pressure curve will vary by the actual energy composition of the propellant (single or double based), deterrents used, temp, etc. If only it were so simple as do "x", get exactly "y". I haven't found anything better yet than making up the loads, and letting the target and chronograph tell me if all the stars have aligned.

 

[rcmodel]

found that the volume of the same powder for the same bullet is lower in the short barrel load tables.

I don't have the latest Speer manual to see what you are seeing.

But, one possible answer is Speer has a line of "Short Barrel" bullets designed to expand at lower velocity then the heavier jacketed Gold Dots.
They have bigger & deeper HP cavities, and thinner jackets.

They are loaded down to perform properly at the design velocity possible from a short barrel.
Loaded as hot as is possible with Gold Dots in a longer barreled .44 Mag, they would very likely blow up and not perform properly.

rc

 

[EddieNFL]

That's a pretty good analogy, RC. I'll have to remember that.

I was assuming that was the case because once the bullet left the barrel, all the powder that had not yet burned was useless (except for a cool light show).

Forgotten where I read this, but (someone) stated the powder is all burned in the chamber/barrel and the flash is gas exiting the muzzle. That sort of goes along with CCWs statement about peak pressure. Don't know how true that may be, but I though it was interesting.

 

[rcmodel]

Most of the muzzle flash is super-heated gas coming in contact with the oxygen in the air.

But there is also unburned powder ejected with any powder burn rate.

That can be proven by the occasional indoor range fire when they neglect to sweep the floor once and a while.

Even the Glock factory test range suffered a powder fire a few years ago that burned three employees pretty bad.

However, unburned powder grains get blown out of the muzzle regardless of the burn rate of the powder used. Even .22 RF suffers from it to some extent and that's some fast powder right there.

rc

 

[Galil5.56]

Even .22 RF suffers from it to some extent and that's some fast powder right there.

VV 3N37 is used for some 22LR ammo... I think a lot of 22 LR ammo uses propellant a lot "slower" than many people think, especially "Stinger" types. Regardless, keeping a range clean to avoid "blow by" fires is wise indeed.

 

[counterclockwise]

found that the volume of the same powder for the same bullet is lower in the short barrel load tables.

Here is an example from Hodgdon's data that shows the above becomes an exception and not the rule.
(the short and the long of it, one might say)

.357 mag Pistol:

140 GR. HDY XTP Hodgdon H4227 .357" 1.590" 16.2 1541 33,100 CUP 18.0 1685 42,600 CUP
140 GR. HDY XTP Hodgdon H110 .357" 1.590" 17.1 1597 28,400 CUP 19.0 1762 40,900 CUP
140 GR. HDY XTP Hodgdon HS-6 .357" 1.590" 9.5 1411 35,800 CUP 10.5 1539 43,000 CUP
140 GR. HDY XTP Hodgdon Universal .357" 1.590" 6.5 1218 34,800 CUP 7.0 1299 40,200 CUP
140 GR. HDY XTP Hodgdon HP-38 .357" 1.590" 6.5 1219 30,800 CUP 7.7 1378 41,900 CUP
140 GR. HDY XTP Hodgdon Titegroup .357" 1.590" 6.3 1262 35,600 CUP 7.0 1376 41,900 CUP

.357 mag Rifle:

140 GR. HDY XTP Hodgdon H4227 .357" 1.590" 16.2 1798 33,100 CUP 18.0 1930 42,600 CUP
140 GR. HDY XTP Hodgdon H110 .357" 1.590" 17.1 1836 28,400 CUP 19.0 1997 40,900 CUP
140 GR. HDY XTP Hodgdon HS-6 .357" 1.590" 9.5 1497 35,800 CUP 10.5 1613 43,000 CUP
140 GR. HDY XTP Hodgdon Universal .357" 1.590" 6.5 1282 34,800 CUP 7.0 1356 40,200 CUP
140 GR. HDY XTP Hodgdon HP-38 .357" 1.590" 6.5 1324 30,800 CUP 7.7 1447 41,900 CUP
140 GR. HDY XTP Hodgdon Titegroup .357" 1.590" 6.3 1325 35,600 CUP 7.0 1425 41,900 CUP

 

[ranger335v]

"I was thinking the load for the same bullet between a 7 1/2" barrel and a 2 1/2" barrel would use different powders to optimize velocity. "

That isn't true, any load that develops the highest velocity in a long barrel will do so in a shorter one. Not writing a book on it but what matters for velocity is the area below the time/pressure curve until the bullet exits. No barrel length will change that.