Load Pressure Long barrel, Short Barrel

[gradog]

It seems to me that a 50 cal barrel 34" long with a 60grn load of FFF would develop less pressure and fps than a barrel 38" long. Also if the barrel 34" has a max load of say 100grns a 38" barrel max load would be less. Is this wrong. Is there any info out there on the subject.

 

[Zonie]

I'll give my take on the question.

First off, the maximum pressure is in the area of the breech so the length of the barrel doesn't have anything to do with it.

If we are talking about the maximum pressure at the very moment that the ball exits the muzzle then the 34 inch barrel would have a slightly higher pressure than the 38 inch barrel.

That is only because as the ball is moving down the barrel the pressure drops for every inch of travel before the ball reaches the muzzle.

Because the 38 inch barrel is 4 inches longer, the pressure in it would drop some during those last 4 inches before the ball reached the muzzle.
This would make the pressure at the moment the ball exits the muzzle slightly lower in the long barrel than the pressure was in the shorter barrel when the ball left its muzzle.

Even though the pressure is dropping during those last 4 inches of extra barrel length, the pressure still has a lot of energy in it so it will continuing to accelerate the ball while the shorter 34 inch barrel contributes nothing to the speed of the ball as it travels 4 inches outside its shorter barrel.

Put another way, the longer barrel will usually give a higher velocity to the ball.

There is a limit to this though.

In tests done by Dixie Gunworks years ago they tested many different powder charges in a barrel to determine the muzzle velocity of each powder load.
They then cut off 2 inches of the barrel and ran the same tests again. They continued to shorten and test the barrel until it became so loud or it had lost so much velocity that further testing would prove nothing.

With very light powder charges, shortening the barrel either did not change the velocity or in some cases it actually increased it a bit.
IMO, this is because the small powder charge had used up all of its energy before the ball exited the muzzle so in these last inches of barrel length, the friction of the patched ball slowed the ball down.

With the larger powder charges, which had more energy, shortening the barrel always reduced the velocity of the ball.

Likewise, increasing the powder charges always increased the velocity of the ball in both short and long barrels.

While all of this is interesting to think about, remember, accuracy is as important or more important than velocity and because each barrel is different, each barrel will have favorite powder charges that work better than other powder charges in the same barrel, regardless of the barrel length.

 

[hunts4deer]

There is a physics law (Boyle's law) regarding compressible gasses that Pressure x Volume = Constant. In other words when the bullet has travelled twice as far down the barrel, only half of the pressure remains. Likewise, when the bullet is about to exit a 38 inch barrel, the pressure is one tenth that it was when the bullet was 3.8 inches from the breech.

This is not exactly the case, because the powder doesn't burn and turn fully to a gas instantly, but continues to burn and add pressure. But by then the pressure is dropping because of bullet travel down the barrel increasing volume. But at any rate, the maximum pressure in the barrel happens just as the bullet starts to get moving.

 

[matt denison]

Well said Zonie.

Friction is key in a long (sometimes even short) barrels.

Years ago I read of an experiment in IIRC Guns & Ammo Magazine where they took a .45-70 rifle with a 38" barrel and shot it several times to establish an average velocity. The SD was under 20. Then they proceeded to lop off two inches and found that the velocity was virtually unchanged. Two more inches and the vel. went UP. Two more inches and it went UP again and continued to until they reached 30". The gain in vel. was theorized as reduced friction. The last 8 inches was doing nothing except slowing the bullet and adding some lengthened sight plane. Spinning a bullet in rifling sucks up a lot of energy and adds a lot of friction.

 

[longcruise]

Wonder if that 45-70 test was with bp or modern powder?

This slowing down is also noted in .22 rimfire rifles, but I've no idea where the point break is.

 

[Robert Egler]

I read something about .22s recently where someone had done this experiment with a rifle himself, and if I recall the break point was around 18 inches, although IIRC the interesting thing is that muzzle velocity didn't significantly drop off after that until the barrel was 12 inches or shorter, it was a non-linear change in velocity vs barrel length. I'm not sure why that would be the case.

I'll see if I can find the article.

 

[Bill Hall]

Here's a real world example of what Zonie was explaining. I built my daughter a cute little 26" .54 rifle. There have been times when I shot my 34" .54 along side of her. With both of us using 80 grains, my rifle has a respectable "boom", whereas her's makes an ear splitting "crack", much louder than mine. Quite a noticeable difference in loudness. I'm assuming the this difference is due to the shorter barrel having higher pressure when the ball is exiting, as compared to the longer barrel and lower "used up" exiting pressure. Bill

 

[gradog]

Thanks guys. I had it all back assward. I thought that the pressure built as more powder burned. Light dawns on marble head.

 

[nkbj]

It seems to me that a 50 cal barrel 34" long with a 60grn load of FFF would develop less pressure and fps than a barrel 38" long. Also if the barrel 34" has a max load of say 100grns a 38" barrel max load would be less. Is this wrong. Is there any info out there on the subject.

Time at pressure due to barrel length can play a part in firearms but it being a significant factor usually only shows up in reloading auto pistol cartridges where a small linear increase in barrel length is a significant per cent increase. Then sometimes the soft brass can be seen to have been subjected to "more" pressure and a longer barrel to give more flattening of the primers. So, my thought is that more time equals more transfer of force to the pressure containing envelope whether the arm is a muzzleloader or of younger design. But is it really pertinent to pressure concerns in a front stuffer? Beats me.

 

[Dphar]

It seems to me that a 50 cal barrel 34" long with a 60grn load of FFF would develop less pressure and fps than a barrel 38" long. Also if the barrel 34" has a max load of say 100grns a 38" barrel max load would be less. Is this wrong. Is there any info out there on the subject.

Maximum pressure will be within 6-12" of the breech barrel length is irrelevant.

Dan

 

[Stumpkiller]

Gotta get this in before the Davenport Formula rears it's quirky head.

Back in 1918 the Krupp factory came up with an 8" bore of 195 calibers (barrel 195 x longer than the bore), or 130 feet +/-. They stuffed 400 POUNDS of black powder in the breech and, when touched off, the 264 pound shell exited the muzzle at over 5,200 fps, passed into the stratosphere 25 miles up, had to take the earth's rotation into effect for the aiming coordinates (35 years later the same tables were used in the V-2 project. Smart fellers in 1918!) and, 170 seconds later, hit the city of Paris. The next sequentially numbered shell would be 1/50th of an inch larger in diameter to account for barrel erosion. After 65 shots the bore was 9.4" and the gun sent back to Krupp for reboring.

What does this prove? That would be the equivalent of a .50 cal rifle with a barrel of 8 feet and a 270 grain charge! Don't try this at home, kids. You can use a long barrel and not have "excess" if you burn a LOT of powder. Certainly not efficiently, but more than enough is always sufficient.

 

[Bill Hall]

Thanks guys. I had it all back assward. I thought that the pressure built as more powder burned. Light dawns on marble head.

Like Dan said, the pressure builds only a few inches, then declines gradually as barrel distance increases. If you could see it on a pressure/distance graph, the pressure would spike up abruptly in the breech area, then slope down as the bullet traveled down the barrel. That's why octogon-to-round, or tapered barrels are often used. The thicker metal is only needed in the breech area, to contain the pressure spike. The thinner barrel walls past the breech are strong enough to handle the declining pressure. Bill

 

[zimmerstutzen]

Black powder has a longer pressure curve than smokeless. It builds slower. In the Guns and Ammo test mentioned above, if it was done with smokeless, it has absolutely no relevance to black powder shooting.

As long as there is pressure behind the ball to keep accelerating it behind the ball, velocity will increase. Some people think that black powder is entirely burned within a few inches of the breech. It isn't. if it was there could never be any flame in muzzle flash. There comes a point in increasing powder charges that the fouling left behind increases significantly over lower charges. As it was explained to me many years ago, black powder burns more completely under pressure. When the pressure is releases, the burning is less complete, leaving more fouling. So if the ball exits the muzzle while significant burning is still taking place the fouling increases dramatically.Any body that ever tried shooting unpatched balls knows the fouling builds up much much faster. a factor of powder burning under less pressure. Lyman published a book with pressures for various loads in various length barrels and the figures when comparing pressures and velocity don't necessarily correlate to higher velocity in the shorter barrels. If all the powder is burned in the first 6 to 12 inches, then pressure measurements between long and short barrels should be equal. They simply aren't. Especially given the copper crush method used in their tests. It didn't measure pressure at a particular down barrel point, but just measured max pressure. If as some surmise, the pressure is always equal in the first 6 to 12 inches, the measurements would have to be equal as well. Since the maximum pressure measurements are not equal, then powder must be continuing to burn beyond that point in the balls travel down bore.

 

[matt denison]

I tried to reply to Marmotslayer yesterday, don't know where it went.. :idunno: still floating in cyberhell I guess.

The gun in that experiment was a trapdoor, BP cartridge gun.

I don't think anyone is saying that powder doesn't continue to burn after the presure spike in the first 6 - 15 inches, only that the explosion has peaked and presure recedes rapidly after that. It would depend on powder charge volume and bore size and weight of the projectile. Also how well the projectile fits the bore for a more efficient burn but remember that too tight and you start the friction thing earlier in the bullets travel down the bore.

:surrender:

 

[zimmerstutzen]

Just as a for instance: 60 grains in a 45 caliber barrel
velocity pressure
28 inch, 1593 ft/sec 5,660 LUP
40 inch 1755 ft/sec 9,140 LUP
43 inch 1831 ft/sec 9,720 LUP

a little more velocity, but substantially more max pressure in longer barrels. I think this disproves the idea that max pressure is reached in the first 12 inches of bore. There is even a pressure increase between the 40 inch and 43 inch bores.

compare the figures for the same charge in a 54 caliber barrel
velocity pressure
28 inch 1250 ft/sec 4,860 LUP
34 inch 1359 ft/sec 6,800 LUP
43 inch 1424 ft/sec 7,000 LUP

the same trends are apparent.

What possible explanation can there be for increasing pressure in longer barrels, except that the same charge is still burning and increasing pressure when the ball leaves the muzzle? (There does come a point in the shorter barrels that increased powder charges produce only minor increases in velocity)

An interesting thing is that pressures drop between 45 cal and 54 caliber. Considering the larger heavier ball, I would have thought the opposite.

 

[rbseagle]

i agree with you. if pressure built up with longer barells then the barell would have to be thicker on the end than at the breech. modern day center fire rifles the barells are thiner at the muzzel than at the chamber. just my way of thinking. :hmm:

 

[zimmerstutzen]

Now ironically for a 50 cal in the Lyman tests, the 60 grains of fffg produced a slightly different curve

26 inch 1461 7800 LUP
28 inch 1460 8100 LUP
32 inch 1554 7900 LUP
43 inch 1611 7800 LUP

It still shows an increase in pressure between 26 to 28 inch barrels. But still disproving the idea that powder pressure curves reduce rapidly after 12 to 15 inches. the max pressure continues to increase even after the ball has traveled 26 inches.

if we increase the powder to 80 grains in a 50 cal.

26 inch 1690 11,000 LUP
28 inch 1692 11,400 LUP
32 inch 1777 11,400 LUP
43 inch 1854 11,700 LUP

still shows increasing pressure after 26 inches of bore.

 

[zimmerstutzen]

Hmmmm. no rebuttal from those who mistakenly say the pressure peaks at 15 inches. Guess that settles it.

 

[matt denison]

Hmmmm. no rebuttal from those who mistakenly say the pressure peaks at 15 inches. Guess that settles it.

Sorry that I couldn't log on later last night to continue this conversation.

Your figures are taken (I would guess) from a book. Lyman perhaps? There is nothing to say that presure is increasing as the ball passes down the barrel. Only that there is more velocity from the longer barrels. The presure shown isn't measured at, say 6" from the muzzle is it? As the ball passes down the barrel the hot gasses are expended and allowed to expand into an ever increasing volume of area. In my personal experience it is intuitively obvious that although a longer barrel will allow higher velocity the increase diminishes with inches of length to the point of little or no advantage.

Not argueing, this is all very interesting to me. Always willing to learn something or anything about internal balistics. Your chart is interestiing, I have to look it over again and process.

 

[Dphar]

Black powder has a longer pressure curve than smokeless. It builds slower. In the Guns and Ammo test mentioned above, if it was done with smokeless, it has absolutely no relevance to black powder shooting.

As long as there is pressure behind the ball to keep accelerating it behind the ball, velocity will increase. Some people think that black powder is entirely burned within a few inches of the breech. It isn't. if it was there could never be any flame in muzzle flash. There comes a point in increasing powder charges that the fouling left behind increases significantly over lower charges. As it was explained to me many years ago, black powder burns more completely under pressure. When the pressure is releases, the burning is less complete, leaving more fouling. So if the ball exits the muzzle while significant burning is still taking place the fouling increases dramatically.Any body that ever tried shooting unpatched balls knows the fouling builds up much much faster. a factor of powder burning under less pressure. Lyman published a book with pressures for various loads in various length barrels and the figures when comparing pressures and velocity don't necessarily correlate to higher velocity in the shorter barrels. If all the powder is burned in the first 6 to 12 inches, then pressure measurements between long and short barrels should be equal. They simply aren't. Especially given the copper crush method used in their tests. It didn't measure pressure at a particular down barrel point, but just measured max pressure. If as some surmise, the pressure is always equal in the first 6 to 12 inches, the measurements would have to be equal as well. Since the maximum pressure measurements are not equal, then powder must be continuing to burn beyond that point in the balls travel down bore.

I have seen the BP curve for a 50-140. This was done through a NRA Grant to study duplex.
It has been written by a person who did some pressure testing with BP that in some ways its like a fast pistol powder with its rapid initial acceleration and like a much slower powder due to its longer pressure curve.
Pressure is always highest just as the bullet starts to move or during initial acceleration. As the bullet moves away it increases the bore volume at an increasing rapid rate. Progressive nitro powders are designed to keep the pressure high for longer periods. But even then a cartridge like the 5.56 nato develops about 55000 psi chamber pressure but has fallen to 13000-13600 at the port.
If smokeless is fired in too short a barrel even a relatively fast powder like Unique will produce almost no velocity. A standard velocity load of Unique fired from a SAA Colt 44-40 with the barrel removed will hardly dent a 3/4" particle board.
Loaded with blackpowder the bullet will punch through the first board and seriously dent the second. Across a card table it would be a argument stopper.
So like the statement above BP has some attributes of a fast powder and some of a slow powder. I.E. it does both.

The initial acceleration of BP is pretty violent and will actually upset the bullet before it starts to move. This is why the various BP bullets can be used when under bore size. The minie ball is an example. Eventually, about the time the Minie became obsolete, it was found that the hollow base was not needed either. BP will bump up long bullets as hard as 1:16- 1:14.
Round balls, the smaller ones at least, have far less inertia and don't bump up much if at all it would seem.

I also know that 37 gr of FFF in a 38-40 case will move a 180 gr bullet out of a 4 3/4" Colt Bisley at almost 1000 fps.

Unpatched balls foul more because the patch is not there the wipe the bore between shots and some lubes will reduce fouling build upand/or soften the fouling.
A large percentage of BP does not burn and this is where the fouling comes from. Some is deposited in the bore some is ejected from the muzzle with the projectile. The
The flash at the muzzle is made up of solid remnants of combustion, fouling at near 2000 degrees and hot gases.
Dan