Faster Burn = Lower Pressure?

[cyclone]

Very confused about the relationship, esp. where peak pressures come into play (faster powders tend to have sharper high-pressure peaks, apparently ? ).....and I know this is a newbie-esque question, so thanks in advance !

 

[H4831]

Very confused about the relationship, esp. where peak pressures come into play (faster powders tend to have sharper high-pressure peaks, apparently ? ).....and I know this is a newbie-esque question, so thanks in advance !

Fast burning powder raises pressure very rapidly.
Always remember this, The most efficient, and safest powder to use, is a powder that fills the case to about 95%, or more, of it's capacity, for a normal, full power load.

A powder that burns so fast as to give full pressure for the cartridge, while only filling the case to half, or less, of capacity, is an unefficient powder and very dangerous to use. Extremely dangerous if a double charge is accidentally put in, but also dangerous because slightly more powder may give a dangerous pressure spike.

 

[CeeZer]

Ok. Theoretically.. Two different powders with exactly same burn rate but different burning temp will do?.. Cause different pressure spikes. It's the temp that expands gas (air) creating extra volume, and if volume remains the same = extra pressure.
As powder gets ignited, it raises temp inside the case, that creates extra volume=pressure and that pushes bullet out (just like if you shake champagne bottle and loosen the cork). Bullet starts moving down the barrel and thus there is extra volume behind and between it and the case, but powder is still burning too and still creating even more volume. So, peak pressure may take place either when bullet is still inside barrel or when bullet leaves barrel before peak pressure is still to be reached. So, in some cases, longer barrel may allow for higher bullet velosity.

In real life, the rule of thumb is, the heavier the bullet is the slower powder one has to use. That said, I presumed barrel (length) was the same.

 

[cyclone]

Just a bit confused b/c a powder like Trail Boss apparently burns pretty fast , but is recommended for old-timey (and antique) revolvers ?...

 

[CeeZer]

Old, very old guns were using black powder (there was no such thing as 'smokeless' at the time), which is pretty much an explosive as opposed to modern smokeless powders.

 

[SandRoad]

Just a bit confused b/c a powder like Trail Boss apparently burns pretty fast , but is recommended for old-timey (and antique) revolvers ?...

Yes, because it uses very small charges. Another relationship is the amount of powder. More powder = greater pressure.

The relationships are very complex, and all that is being discussed here are generalities. It is not a nice, neat one to one relationship.

 

[Boomer]

Cartridge capacity, caliber, and bullet weight must all enter the equation of the propellant's appropriate burning rate. Lets consider various sizes of .30 caliber cartridges . . .

.32 Auto - 70 gr bullet - 2.0 grs Bullseye

.30 Carbine - 110 gr bullet - 14.0 grs H-110

.30/30 ...110 gr bullet - 29.0 grs IMR4198
.......... ..150 gr bullet - 28.0 grs R-7
............ 170 gr bullet - 30 grs IMR 4064

.30/06....150 gr bullet - 53 grs IMR 4064
.............180 gr bullet - 60 grs H-4831

.30-378...180 gr bullet - 115 grs H-870
..............240 gr bullet - 102 grs H-870

From this we see that small capacity cartridges with light bullets require a faster powder than larger cartridges with heavier bullets. If we choose too fast a powder for a cartridge or bullet weight, we will observe excessive pressure before our optimum velocity is reached. If we choose too slow a powder, the cartridge will not hold enough powder in order to produce the pressure required to reach the optimum velocity.

 

[Ganderite]

"So, peak pressure may take place either when bullet is still inside barrel or when bullet leaves barrel before peak pressure is still to be reached"

No. Peak pressure is reached in the first couple if inches. After that the extra volume of space behind the moving bullet causes pressure to drop.

A progressive powder starts to burn slowly but burns faster as it is consumed. This is an attempt to try to take advanage of the larger space behind the moving bullet.

Most powders burn slower as they are consumed. It is a function of exposed area. Think of a burning log. At the beginning there is a large area giving off gas. later, when the log is smaller, there is less surface area.

The correct powder charge of both a fast and slow powder can have the same peak pressure, but the slow powder (more of it in the case) will sustain more pressure longer and give higher velocity. The backside of yhe pressure curve of a fast powder is very steep, whereas the backside of a slow powder pressure curve is long and gentle. Velocity is a function of the area under the curve, peaks being the same.

Powders burn reliably at a fairly high pressure, so if you want a lower velocity (cast bullet) with good accuracy, you should use a faster powder instead of less of a slower powder.

 

[mwjones]

I've wondered about this as well... MY lyman cast handbook lists loads of anywhere from 9 to 13 grains of red dot, green dot, 700x, unique for .303 british. These are pretty fast powders i thought, so i fired up quickload, and the max charges are showing pressures around the same as minimum charges of rifle powder like 4895 and bl-c(2). scary to think what a double, triple, or even quadruple charge (think 40 grains of 700x) would do to my old enfield :O

 

[ATOM]

It's the temp that expands gas (air) creating extra volume, and if volume remains the same = extra pressure.
As powder gets ignited, it raises temp inside the case, that creates extra volume=pressure and that pushes bullet out (just like if you shake champagne bottle and loosen the cork).

Yes and no.

The incandescent temperatures definitely raise the pressure, IAW Charles' Law.

However, a very small volume of solid (the propellant) is changed to a very large volume of gas. In the process, it generates heat, raising the pressure still more.