Pressure Changes by Barrel Length

[Walkingeagle]

Can someone please enlighten me. I have a copy of the Lyman Blackpowder Handbook & Loading Manual, second edition, copyright 2001. On page 226 (pictures are below) it shows loading, velocity and pressure data using Goex 3fg with 24” barrel, on page 237 same data is shown but with 28” barrel.
Why is the pressures generated so much higher with a shorter barrel with all else being equal?
Walk

 

[Zonie]

My best guess is all else wasn't equal. Perhaps the 24" barrel was a little smaller in size or rougher in the bore finish than the 28" barrel was.
Either of these things can raise the breech pressure in a barrel when the powder fires.

Then again, it might be due to some mystic, unknown power at work behind the scenes?

 

[BrownBear]

My guess too, because peak pressure is going to be in the first few inches of barrel and dropping after that before the ball ever leaves the barrel. Meanwhile the little chronograph work I've done has shown that I can run velocity (and by extension, pressure) up and down the scale with changes in patch thickness or ball diameter.

 

[Col. Batguano]

It would be nice if someone made an inexpensive pressure measuring device, but there isn't one that I know of. Instead, we're left to those with the sophisticated labs to do it for us. But, just because they have sophisticated equipment that doesn't necessarily mean that those using it are sophisticated themselves.

With cartridge guns we have indicators like sticky extraction, flattened or cratered primers, or brass swaged in to the extractor groove as clues to excess pressure. With cap guns sometimes the cock gets blown back. With a flint gun all you have is the size of the smoke plume out the touch hole, which is dubious at best to measure pressure.

 

[Grenadier1758]

And with a flint gun, if you are concentrating on the sight picture, you won't see the pan flash other than to know it happened because the smoke from the barrel obscured the target.

 

[Elkeater]

It would be nice if someone made an inexpensive pressure measuring device, but there isn't one that I know of.

As BB alluded to, a chronograph works. Velocity and pressure are directly related.

 

[Grimord]

There is a new device available to hand loaders that measures pressure. It has a transducer that sticks to the barrel above the chamber on suppository guns. Don't see why it wouldn't work on a BP gun with the transducer placed just in front of the breech plub. Hopefully some one else in this forum will chime in regarding this device. I read about it in "Handloader" magazine, but i can't remember when. CRS disease.

 

[bang]

My guess is the extra 2 inches of barrel and the drop of pressure in that length.
To get same 40gn FFG pressure at muzzle of a 24 inch in a 28 inch you use 75gn
FFFG 80gn.
That really shows you how drastic the pressure drop is in relation to powder measure when it takes right at 2x the powder to achive pressure in 2 more inches of barrel.
I read somewhere that the max effective expansion of black powder has pretty severe limits when your looking for pressures and speed to achive flater trajectory. It's not that you can't achive long shots it just means it will have a trajectory arc like a rainbow.

 

[Boomerang]

I would think the maximum pressure would be what it takes to get the projectile started moving down the barrel. From there on out I think it would start dropping off.

 

[Elkeater]

At that point where projectile is just starting to move, powder is still burning, pressure and velocity climbing.

 

[BrownBear]

To get same 40gn FFG pressure at muzzle of a 24 inch in a 28 inch you use 75gn
FFFG 80gn.
That really shows you how drastic the pressure drop is in relation to powder measure when it takes right at 2x the powder to achive pressure in 2 more inches of barrel.

I can't follow that at all. Care to explain further?

 

[SDSmlf]

To get same 40gn FFG pressure at muzzle of a 24 inch in a 28 inch you use 75gn
FFFG 80gn.
That really shows you how drastic the pressure drop is in relation to powder measure when it takes right at 2x the powder to achive pressure in 2 more inches of barrel.

How did you arrive at these pressure numbers?

 

[Col. Batguano]

There is severe and drastic pressure drop as you go down bore. A 30-`06 generates about 50M cup / psi at the breech, but where the gas port is in a Garand, it has to be <5000 for th eop rod, and that's about 23" down bore.

With our ML'ers and peak pressures around 15,000, at the muzzle of a 44" barreled gun I bet at most we're talking about 200.

The net effect of all of this for builders is that once you get past that first barrel lug insert, (or the rear sight) the thickness of the remaining barrel wall under the lug is of lesser and lesser and lesser concern the farther down bore you get. Using the above Garand numbers, my guess is that at the 10" mark, which i where a lot of rear sights are, peak pressure is probably down by 50% or more. I shoot very light target loads most of the time. 18 grains in a 48" barreled 38 caliber, which gives me velocities in the 1060 with a 83 gr. ball (I wanted to stay sub-sonic with it.) . I bet my muzzle pressures are even less than mid double digits. Not much worry about blowing out a front sight that might have been cut in too deep!

 

[bang]

How did you arrive at these pressure numbers?

On the chart.
40gn ff 24" 6800 psi
40gn ff 28" 4200 psi
But if you make the load of a bit over 70gn in 28" you get just over 6300 psi. Looking for 6800 which would be same as 40gn in 24"
If I missed some let me know but that's what it looks like by the charts posted

 

[longcruise]

When the powder ignites and peak pressure is hit, the component response (peak pressure) has no idea how long the barrel is. The only remaining variable having to do with barrel length is the volume of and weight of the air in front of the ball that is resisting movement. So that's something to put a pencil to.

I, like Brown Bear, have witnessed the ups and downs of chronograph numbers with variations in components and barrel condition.

 

[plmeek]

Can someone please enlighten me. I have a copy of the Lyman Blackpowder Handbook & Loading Manual, second edition, copyright 2001. On page 226 (pictures are below) it shows loading, velocity and pressure data using Goex 3fg with 24” barrel, on page 237 same data is shown but with 28” barrel.
Why is the pressures generated so much higher with a shorter barrel with all else being equal?
Walk

Welcome to the real world of laboratory data. I think you've stumbled onto some data flaws that got by the lab technicians and the ballisticians. I'm sure these folks looked at the data in tabular form, but doubt they looked at it in graphical form. If they had, they should have noticed some odd behavior in the 28" barrel pressure data.

Below is the pressure and velocity data for the 24" barrel plotted versus the charge volume. The FFFg pressure data is the "red" curve or the top curve. The FFg pressure data is the "blue" or next to highest curve. As expected, the 3Fg charge produces a higher pressure than the 2Fg charge of equal volume. The velocity for 3Fg is also higher than the velocity for equal volume charge of 2Fg.

This next graph is the pressure and velocity data plotted for the 28" barrel. Note the odd behavior of the "red" or FFFg curve. It inclines or slopes up for the first three points like the 24" barrel data, but after the 60 gr load it starts to flatten out. The difference between the 90 gr load and the 120 gr load is only 300 psi! Anyone that shoots a 54 caliber rifle knows that this can't be correct based on felt recoil. Where the 3Fg curve and the 2Fg curve for the 24" barrel are almost parallel, the two curves for the 28" barrel data appear to intersect at just over 120 gr charge. There certainly appears to be something wrong with this data. It might have been a problem with the piezoelectric equipment.

To confirm this interpretation, I looked at the data in Lyman's 1975 Black Powder Handbook. They only tested 3Fg powder, but that is the data in question in the 2nd Edition book. They also tested four different barrel lengths, 28, 30, 34, and 43 inches. Again, we're fortunate that the 28" barrel data is in both books. The powder used is the older Gearhart-Owen. Pressures were measured with Lead and Copper Crushers, which means individual points or pressures are not directly comparable to 2nd Edition book data, but the trends still are. As can be seen in the graph below, the pressure for all four barrel lengths increase with increasing powder charges.

This appears to confirm that the 2nd Edition book pressure data for the 54 caliber, 28" barrel using 3Fg powder did not accurately measure the breech pressure. I would consider that data unreliable, if not meaningless. I would also question the absolute values of the 2Fg data in the 2nd Edition book for the 28" barrel. The shape of the curve looks ok, but the 1975 data shows that the pressure is the same for the 28" and 30" barrels for nearly all the charges. The 2nd Edition data for the 2Fg charges is showing over 2,000 psi differences between the two barrels in the lower powder charges. With the higher powder charges, the difference in pressure begins to narrow. The fact that the 24" barrel velocities are generally higher than the 28" barrel in the 2nd Edition data suggests that the bore conditions between the two barrels are not equal.

One paper I read online stated that the CUP measuring system has a random error of about 2,000 units while the piezoelectric system has a standard deviation of about 1,300 psi. But there appears to be a systematic error of some kind in the 2nd Edition data. There is something about the bore condition, the loading technique, or the components that wasn't equal when testing the GOEX powders in the two barrels. Looking at the data for the Elephant powder and the Pyrodex powder, the results, both velocity and pressure, are more what one would expect by changing barrel length only.

 

[Col. Batguano]

I agree on the bore condition thing. There is something intuitively wrong, or at least different. There are other variables too, like whether the bore was clean or dirty, ball and patch thickness, type of rifling, lube. A whole bunch of things. Is one gun a cap gun and the other a flint? And the other variable; operator competency.

The mpst relevant thing about the data is the relationships between loads, pressures, and velocities as posted for the same gun. we can be reasonably certain that conditions were pretty much the same from load to load there, if not the raw number data integrity.

 

[bang]

Perhaps the leveling and or meeting on the graphs is and indicator of the pressure limitations of black podwer in this use. Given the max pressure initally is at ignition then disapated rapidly.
Wish I had saved it but I read somewhere that experiments with artillery long ago when black podwer was the only used that the limits of max range vs pressure were proptionat between diameters.
This of course is in relationship to only sraight bore and before any more confined bore with larger charge chamber than bore. This was a detailed explanation that yes you can get bigger bang with more powder but the fall of pressure was that once you get to a certain point thus range did not change enough to use any larger charge than what produced the best range. That black powder had its limits.
Now with modern propellants there is a vast difference in use due to larger charge vs bore and the pressures being beyond what black powder can produce and increased expansion time. The max range and flater trajectory and the limits are way beyond those limits of black powder but none thes less has limits also. I seem to remember reading some very large ship cannons that could shoot miles with x amount of powder but if you added an extra 5, even more, pounds or couldn't get any better range or trajectory . Granted everything has its limits.

 

[plmeek]

Wish I had saved it but I read somewhere that experiments with artillery long ago when black powder was the only used that the limits of max range vs pressure were proportional between diameters...This was a detailed explanation that yes you can get bigger bang with more powder but the fall of pressure was that once you get to a certain point thus range did not change enough to use any larger charge than what produced the best range. That black powder had its limits.

Yes, for BP cannons, there is an optimum powder charge that gives maximum velocity and range. Go to Prop IV near the bottom of this webpage for the calculations. It says, "So the length of charge for maximum muzzle velocity is 1/2.72 of the length of the barrel." In the case of the 28" barrel, the optimum powder charge would be 28/2.72 = 10.3 inches long. We would never load a 10" column of powder in a muzzleloader rifle, so in a practical sense there is no optimum powder charge for a maximum velocity in a muzzleloader rifle.

No, I suspect there is a problem with the GOEX 3Fg data with the 28" barrel. It shouldn't bend over like that.

I generated graphs for the Elephant powder data and the Pyrodex powder data. None of the pressure curves bend over like the questionable data for the 3Fg in the 28" barrel.

First the Elephant powder in the 24" barrel. The pressure curves are similar to the GOEX data in this barrel, but they are lower. There are also a couple of kinks in the 2Fg curve. The velocity curves are shaped as expected and lower than the GOEX velocities in this barrel. This matches with what I have heard about the Elephant powder. It's not as powerful or efficient as GOEX. It is also interesting that the difference between the 2Fg and 3Fg velocities are not as great as for the GOEX powders.

The Elephant powder in the 28" barrel. The pressure curves almost intersect at the lowest charge volume and do intersect between 110 and 120 grs. Not the expected behavior, but it is worth pointing out that the Elephant 3Fg powder in the 28" barrel does not bend over like the GOEX 3Fg data.

On to the Pyrodex data, this is for the 24" barrel. I chose to compare the Rifle/Shotgun granulation to the Pistol granulation thinking it would be a relative comparison for the 2Fg to the 3Fg black powder. Pyrodex doesn't appear to burn the same as real black powder as the velocity curves almost overlap each other.

The Pyrodex data in the 28" barrel. Again, the velocity data for the two granulations are almost the same. The pressure curves look more like some of the GOEX and Elephant curves in the 24" barrel.

I'm not going to show the graphs, but the velocity curves for Pyrodex RS and Select also overlay each other. In these two barrels, it doesn't appear to matter which granulation of Pyrodex one chooses to shoot. The velocities are pretty much the same for all three.

Looking at all these graphs, it appears that measuring consistent pressure data, even under laboratory conditions, is difficult to do.

Lyman had this to say about their procedures in testing:

The data listed in this section has been tested by Lyman technicians under controlled laboratory conditions...
The velocity and pressure data shown was produced using five shot test strings. The test barrels were cleaned between each five shot string. We felt that this would best duplicate "real world" shooting conditions. All pressure data collected was with the use of piezo electronic equipment and is measured in actual pounds per square inch (PSI). Standard black powder barrels were modified to accept a pressure transducer and were connected to a peak pressure meter.

I'm not sure what to make of this pressure data. Theoretically, one shouldn't see a difference in peak pressure with different barrel lengths, but in this data we do. The internal dimensions of the two barrels may be different and/or the finish or roughness of the barrels may be different. Too bad they didn't conduct the test by shortening the same barrel. But even with these differences, I would expect more consistency between the shapes of the curves in the same barrel with the different powders.

If the pressures are real, meaning the piezoelectric equipment was working right and measuring actual pressures accurately under controlled conditions and still getting this much variation, it makes me wonder what type of pressures we are getting in the field?

 

[voltigeur]

or quite simply a powder that is wetter than the other