I was reading the post on porting a barrel and thought I should post my experience because it may help others, along with intuition in figuring out the spacing and how many vents to drill when porting a barrel to lower velocity.
A while back I was looking at ways to selectively vent barrels to achieve alternating low and normal velocity and the two most notable findings (or at least what I remember) are below:
The first is not surprising and that is for a given diameter vent the effect on velocity increases from zero at the muzzle to a maximum value if drilled just in front of the chamber, so the closer to the chamber the vent the greater the decrease in velocity, I don’t remember if there was liner relations with distance and normalized velocity (vented velocity / un-vented velocity) or an exponential one.
The second is if you drill a line of identical vents in the barrel and thread each for a plug you can measure the normalized velocity for each when only one is open. If you then open multiple vents the normalized velocity will be equal to the product of the normalized velocity of the individual vents. When I found this relationship I then drilled additional vents in between the first five closest to the chamber and the relationship was the same with the measured velocity agreeing with the predicted velocity to within less than 5% difference, if I remember correctly.
Basically if you drill one vent and the velocity is .9 of the un-vented velocity and you drill and identical vent at the same distance the vented velocity with both vents open will be .9 x .9 = .81 of the normal velocity. If you then drill a vent closer to the chamber and the new velocity (with all vents open) is .6 of the un-vented then the new vent on its own produces a normalized velocity of = .6/.81 = .74. You can use that info to estimate what the drop in velocity will be for a vent drilled at a distance in between the third and first two vents or what the velocity will be if you drill a forth vent opposite the third vent (.74 X .6 = .44)
I’ll have to look up my old data and see what the relationship between distance from chamber and normalized velocity was for vents at equal increasing increments from the chamber was. I did this 15 years or so ago and I am not sure where my old cellophane viewgraphs and notebooks are.
Jeff
My expericne venting/porting barrels to decrease MV
My expericne venting/porting barrels to decrease MV
Last edited by widlin1 on Sun Apr 25, 2010 11:08 am, edited 1 time in total.
- Machine Gun Matt
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Very interesting as I have plans to work on my spanish destoyer 9mm carbine and want to vent the barrel in such a way as to make the off the shelf ammo (115 thru 147) stay subsonic but give maximum velocity - did you drill your ports at 90 degrees to the bore axis or did you ever try slanted ports?
"Trying to tax yourself into prosperity is like standing in a bucket and trying to pick yourself up by the handle." - Winston Churchill
I also did some experimentation with porting and came to the following conclusions.
The physics and the math behind the expanding gases of a gunshot are so complex, with so many variables, that it becomes meaningless to almost everyone here. Simply move the location of the port, increase/decrease the size of the port, or change the ammunition (pressure), and either the sound suppression changes, or the velocity, or both.
What is the easiest to understand by most gun enthusiasts’, is that the shorter the barrel the lower the velocity. By that, I don’t really mean shortening the barrel, but port the gun aggressively enough that the velocity of the bullet in the barrel stops accelerating at a certain point in the barrel. Keep in mind there is also a relationship between porting and sound suppression.
Let us say that you didn’t want the velocity from certain ammo to be less than that fired from a 5 inch pistol. I would then port the barrel at 5 inches from the chamber. If the ports were large enough to vent the gas and stop acceleration, then your velocity target would be met, but not necessarily your sound suppression target. (This followed one of my experiments.)
I ported a barrel 5 inches from in front of the chamber, and found the velocity was “Xâ€
The physics and the math behind the expanding gases of a gunshot are so complex, with so many variables, that it becomes meaningless to almost everyone here. Simply move the location of the port, increase/decrease the size of the port, or change the ammunition (pressure), and either the sound suppression changes, or the velocity, or both.
What is the easiest to understand by most gun enthusiasts’, is that the shorter the barrel the lower the velocity. By that, I don’t really mean shortening the barrel, but port the gun aggressively enough that the velocity of the bullet in the barrel stops accelerating at a certain point in the barrel. Keep in mind there is also a relationship between porting and sound suppression.
Let us say that you didn’t want the velocity from certain ammo to be less than that fired from a 5 inch pistol. I would then port the barrel at 5 inches from the chamber. If the ports were large enough to vent the gas and stop acceleration, then your velocity target would be met, but not necessarily your sound suppression target. (This followed one of my experiments.)
I ported a barrel 5 inches from in front of the chamber, and found the velocity was “Xâ€
I agree with Ron about the complexity of the dynamics in a gun tube and can’t explain the fluid dynamics behind it. The relationship between venting and vent size and position that I noted was for the case of venting a .50 AE to the atmosphere, however I also vented into a shroud and to rotating collars separated by ½ inch ID tubes that allowed vents just in front of the chamber to communicate with vents in the mid section of the barrel. Rotating the collars either exposed the vents to the exterior tubes or sealed them. When open the gas was able to bypass the bullet (because the initial velocity of the bullet is low compared to the velocity that the gas can expand at) re-enter the barrel ahead of the projectile thereby dropping the pressure behind the projectile while increasing the pressure in front of it and the mass of gas that has to be displaced by it. When the projectile passes the second set of vents the system becomes sealed again and venting (aside from what leeks) effectively stops. Using the collars connected by tubes I was able to fire an 8 gram 50 cal sabot with a 5.56 penetrator at 850 fps with the vents open and 2100 fps with the vents closed. When I increased the venting to increase the range of high to low velocity the shot-to-shot velocity variation became high and in-bore stickers became a problem. Below are some JPGs of and old briefing.
Jeff
Jeff
Last edited by widlin1 on Sun Apr 25, 2010 5:09 pm, edited 1 time in total.
That quite similar to a design I want to build - I tried talking to severl SOT's to have built but they all shy'd away from it. I want a monocore baffle stack with a sleeve over the ported barrel that can control the gas release to make it subsonic with just a twist of the wrist.
"Trying to tax yourself into prosperity is like standing in a bucket and trying to pick yourself up by the handle." - Winston Churchill