Vent-holes, pressure, caliber and barrel length

Derringeer · April 13, 2020, 04:35 PM

How much pressure will escape from a vent-hole based on the size of the vent-hole? How much velocity will be lost based on caliber and barrel length regarding the size of the vent-hole?

For example, when the caliber and the gunpowder load is small it makes sense that the vent-hole also has to be small, and the opposite is true with a big caliber and big load like in a cannon. But how do you know when big is too big for a vent-hole? Could a gun with a short barrel, like a philadelphia derringer, use a bigger vent hole than a gun with a long barrel without any loss in velocity since the bullet will exit the muzzle before all powder has been combusted inside the barrel anyway?

arcticap · April 13, 2020, 06:35 PM

I don't think that it's much of an issue since any escaping gases can "usually" be compensated for by adding more powder.
In that respect any loss of velocity would be negligible.
I've never heard of anyone doing velocity testing as a way to test for the answer either.
Once the vent hole has been installed, then it becomes a constant.
If the size of the vent hole were to change then one constant would be replaced with another constant.
Both of them could be compensated for by adjusting the powder charge or by other means such as by loading a wad or a thicker patch.
The vent hole would be only one of several variables that can affect velocity.
As long as any vent hole is of a relatively conventional diameter, then I doubt that it's going to make much of any difference in reality.
Perhaps in theory it could be measured or calculated but not sure why anyone would want to do that or how they would measure any velocity change precisely.
It might be similar to wondering if two projectiles had a weight difference of 1 grain, then how much would that affect their trajectories?
Although it's an interesting question, at some point it becomes too small of a variable for most shooters to worry about.
A patch or bullet can also have more or less lube on it.
The nut behind the trigger probably makes more of a difference concerning where the shot hits.

However, I have heard of long range competition shooters who say they notice a decline in accuracy after their [platinum lined] nipple starts to burn out.
Some discriminating shooters may be concerned enough about it to act to replace it, while more casual shooters wouldn't be concerned since they usually don't shoot enough heavy loads to be able to wear their nipple out.
My answer is geared more for the conventional shooter who is not involved in long range competition, and firing conicals with heavy powder loads at distances at the outermost fringe of their rifle's accuracy.
And it's mostly only those types of shooters who may have an idea of how a slightly enlarged vent or flash hole can potentially affect their score.
Another type of shooter who may notice a larger flash hole is someone who shoots an accurate inline where they have experienced flash hole erosion of their breech plug.
They use a pin or two with known diameters to check the diameter of their flash hole for erosive wear as it occurs over time.
And the vent liner or breech plug is a replaceable part which the dimensions of the old and new flash hole can be directly compared.
I don't know the diameter change figures unless they were looked up, and I don't know whether any velocity differences between the old and the new vent liner can be easily measured.
The amount of erosion is relatively small and the vent liner change becomes a matter of routine.
I'm not sure that it's changed for velocity reasons since the vent liners have a very small flash hole that acts as a flash channel to ignite sub powders with 209 primers.

gwpercle · April 14, 2020, 05:25 PM

Philadelphia Derringer has a nipple with a percussion cap covering the vent hole , when the hammer drops , it ignites the cap , keeps pressure down on the cap covering the hole and ....no pressure escapes. If pressure escaped it would blow the hammer back up and the percussion cap would fly off ...that doesn't happen .
I don't think very much pressure gets by at all .
Test have proven the barrel cylinder gap in a revolver do not let enough pressure out to affect velocity much at all ...and that gap is all the way around the barrel ...not just a small hole .
I wouldn't worry about pressure loss...it can't be much at all .
Gary

woodnbow · April 16, 2020, 09:34 PM

25 feet per second. ~~ Depending...

Derringeer · April 19, 2020, 04:25 PM

Quote:

Originally Posted by gwpercle

Philadelphia Derringer has a nipple with a percussion cap covering the vent hole , when the hammer drops , it ignites the cap , keeps pressure down on the cap covering the hole and ....no pressure escapes. If pressure escaped it would blow the hammer back up and the percussion cap would fly off ...that doesn't happen .
I don't think very much pressure gets by at all .
Test have proven the barrel cylinder gap in a revolver do not let enough pressure out to affect velocity much at all ...and that gap is all the way around the barrel ...not just a small hole .
I wouldn't worry about pressure loss...it can't be much at all .
Gary

I’m curious about the physics, how come that the hammer doesn’t blow back during firing? A combination of mechanical strength of the lock and a small touch hole or something more/else? I wounder what the cylinder gap in a revolver is equivalent to when converted into the size of a venthole?

Driftwood Johnson · April 19, 2020, 06:31 PM

It has to do with how large the diameter of the hole is.

The pressure generated by the charge will be equal in all directions. However the surface area of the rear of a ball or bullet is much larger than the surface area of the 'vent hole'. In a cartridge it is called the flash hole, the opening between the primer pocket and the section of the cartridge that contains the powder.

Anyway, when the charge ignites, the pressure generated pushes out equally in all directions. The pressure exerts itself against large surface area at the base of the projectile and pushes the projectile down the bore. The same amount of pressure, let's say 5000 psi to be conservative, venting out the relatively tiny vent hole does not generate enough force to do much. So the hammer spring is sufficient to keep the percussion cap in place.

With a Cap & Ball revolver it is common for the pressure to rip a cap apart. When the revolver is cocked for the next shot it is common for the broken cap to fall into the action and gum up the works. Not a problem with a single shot firearm.

Also, if the charge is stout enough, the pressure can get high enough that it will overcome the hammer spring and will push the hammer back. I have heard of instances where a Cap & Ball revolver suddenly turned into a fully automatic pistol. The back pressure was enough to force the hammer all the way back, cycling the cylinder, and because the trigger was still held back the revolver fired several shots before trigger pressure could be relieved.

This is why it is always recommended to NOT substitute a lighter hammer spring in a Cap & Ball revolver.

April 13, 2020, 04:35 PM	#1
Derringeer Member Join Date: December 21, 2019 Posts: 36	Vent-holes, pressure, caliber and barrel length How much pressure will escape from a vent-hole based on the size of the vent-hole? How much velocity will be lost based on caliber and barrel length regarding the size of the vent-hole? For example, when the caliber and the gunpowder load is small it makes sense that the vent-hole also has to be small, and the opposite is true with a big caliber and big load like in a cannon. But how do you know when big is too big for a vent-hole? Could a gun with a short barrel, like a philadelphia derringer, use a bigger vent hole than a gun with a long barrel without any loss in velocity since the bullet will exit the muzzle before all powder has been combusted inside the barrel anyway?

April 13, 2020, 06:35 PM	#2
arcticap Senior Member Join Date: March 15, 2005 Location: Central Connecticut Posts: 3,166	I don't think that it's much of an issue since any escaping gases can "usually" be compensated for by adding more powder. In that respect any loss of velocity would be negligible. I've never heard of anyone doing velocity testing as a way to test for the answer either. Once the vent hole has been installed, then it becomes a constant. If the size of the vent hole were to change then one constant would be replaced with another constant. Both of them could be compensated for by adjusting the powder charge or by other means such as by loading a wad or a thicker patch. The vent hole would be only one of several variables that can affect velocity. As long as any vent hole is of a relatively conventional diameter, then I doubt that it's going to make much of any difference in reality. Perhaps in theory it could be measured or calculated but not sure why anyone would want to do that or how they would measure any velocity change precisely. It might be similar to wondering if two projectiles had a weight difference of 1 grain, then how much would that affect their trajectories? Although it's an interesting question, at some point it becomes too small of a variable for most shooters to worry about. A patch or bullet can also have more or less lube on it. The nut behind the trigger probably makes more of a difference concerning where the shot hits. However, I have heard of long range competition shooters who say they notice a decline in accuracy after their [platinum lined] nipple starts to burn out. Some discriminating shooters may be concerned enough about it to act to replace it, while more casual shooters wouldn't be concerned since they usually don't shoot enough heavy loads to be able to wear their nipple out. My answer is geared more for the conventional shooter who is not involved in long range competition, and firing conicals with heavy powder loads at distances at the outermost fringe of their rifle's accuracy. And it's mostly only those types of shooters who may have an idea of how a slightly enlarged vent or flash hole can potentially affect their score. Another type of shooter who may notice a larger flash hole is someone who shoots an accurate inline where they have experienced flash hole erosion of their breech plug. They use a pin or two with known diameters to check the diameter of their flash hole for erosive wear as it occurs over time. And the vent liner or breech plug is a replaceable part which the dimensions of the old and new flash hole can be directly compared. I don't know the diameter change figures unless they were looked up, and I don't know whether any velocity differences between the old and the new vent liner can be easily measured. The amount of erosion is relatively small and the vent liner change becomes a matter of routine. I'm not sure that it's changed for velocity reasons since the vent liners have a very small flash hole that acts as a flash channel to ignite sub powders with 209 primers. Last edited by arcticap; April 14, 2020 at 05:38 PM.

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April 14, 2020, 05:25 PM	#3
gwpercle Senior Member Join Date: November 30, 2012 Location: Baton Rouge, Louisiana Posts: 1,748	Philadelphia Derringer has a nipple with a percussion cap covering the vent hole , when the hammer drops , it ignites the cap , keeps pressure down on the cap covering the hole and ....no pressure escapes. If pressure escaped it would blow the hammer back up and the percussion cap would fly off ...that doesn't happen . I don't think very much pressure gets by at all . Test have proven the barrel cylinder gap in a revolver do not let enough pressure out to affect velocity much at all ...and that gap is all the way around the barrel ...not just a small hole . I wouldn't worry about pressure loss...it can't be much at all . Gary

April 16, 2020, 09:34 PM	#4
woodnbow Senior Member Join Date: December 2, 2012 Posts: 190	25 feet per second. ~~ Depending...

April 19, 2020, 06:31 PM	#6
Driftwood Johnson Senior Member Join Date: January 3, 2014 Location: Land of the Pilgrims Posts: 2,033	It has to do with how large the diameter of the hole is. The pressure generated by the charge will be equal in all directions. However the surface area of the rear of a ball or bullet is much larger than the surface area of the 'vent hole'. In a cartridge it is called the flash hole, the opening between the primer pocket and the section of the cartridge that contains the powder. Anyway, when the charge ignites, the pressure generated pushes out equally in all directions. The pressure exerts itself against large surface area at the base of the projectile and pushes the projectile down the bore. The same amount of pressure, let's say 5000 psi to be conservative, venting out the relatively tiny vent hole does not generate enough force to do much. So the hammer spring is sufficient to keep the percussion cap in place. With a Cap & Ball revolver it is common for the pressure to rip a cap apart. When the revolver is cocked for the next shot it is common for the broken cap to fall into the action and gum up the works. Not a problem with a single shot firearm. Also, if the charge is stout enough, the pressure can get high enough that it will overcome the hammer spring and will push the hammer back. I have heard of instances where a Cap & Ball revolver suddenly turned into a fully automatic pistol. The back pressure was enough to force the hammer all the way back, cycling the cylinder, and because the trigger was still held back the revolver fired several shots before trigger pressure could be relieved. This is why it is always recommended to NOT substitute a lighter hammer spring in a Cap & Ball revolver.