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That’s my own vise. $20 swap meet purchased 20 years ago. 
If you want to lower the pressure in the gas system of a weapon, you increase the volume of the gas system itself. A pig tail or expansion chamber in the gas system increases the volume of the system, and that lowers the pressure of the gas in the system.
It regulates pressure indirectly by its relationship to the dwell time, which is dependent on the length of the barrel after the port. For example, if the port it too small, enough pressure does not build up before the bullet exits the barrel. Conversely, if the barrel is short and dwell time is short, the port needs to be opened larger to allow a faster flow of gas into the system to get the pressure up to the required level before the bullet exits the barrel.
The increase in volume using a pigtail in a gas tube is very small. Expanding the internal diameter of the gas tube will have a larger effect. It’s possible either change will not have a practical effect in pressure or dwell time. Either way, it’s purely theoretical speculation until it’s proven by math and/or the change in results are measured
That’s what I said. Port size controls flow (CFM) which determines how fast the system comes up to pressure, not how much pressure will be allowed into the system. If the system were closed, gas would flow through the port until pressure was equal on both sides. A pressure regulator would only allow gases to flow through until the pressure on the outflow side reached a predetermined level
Enlarging the gas port increases VOLUME, not pressure.
Yes, that makes sense, but if the volume of gas that flows through the port is limited by the diameter, wouldn’t that in turn affect the overall pressure?
In other words, if the pressure in the barrel is 10,000 PSI at the port location, but the gas system is fed by a tiny hole for a very short time, would the gas system actually see the same 10,000 PSI ?
Port size controls volume flow (usually expressed in Cubic Feet per Minute) only. What controls pressure is the BCG. Once the pressure inside the BCG reaches a certain point, it begins moving until exposing the vent holes and dumps the pressure overboard
pressure would be the same, just more gas of that same pressure will flow through.
Can anyone recommend some really good books on internal ballistics and on designing semi auto guns?
I have a pdf of Theory of the Interior Ballistics of Guns, by J. Corner.
(I just recently got it and havent read it yet, but it was recommended.)
Any other titles I should look for?
Great read thank you
There is no light reading material on this. There are a lot of articles in the Journal of Applied Physics,
Fiser, M. and Popelinsky, L. Small Arms
Peter, H. Mechanical Engineering. PRINCIPLES OF ARMAMENT DESIGN
Hayes, J., T. Elements of Ordnance
lots of papers from these guys -> http://www.unob.cz
Thanks, ZRH. I’ll have a look for some of those titles.
As I understand it…
According to Boyle’s Law, in any closed system, volume and pressure of gas are inversely proportional. The gas system of the AR-15 is a closed system until the bullet exits the muzzle, at which time the pressure within the gas system rapidly drops to atmospheric. Gas port diameter really controls resistance of gas entering the gas tube, that is how quickly it allows the gas tube to fill and the gas to reach the carrier key (gas lag time). The volume in the system is dependent on barrel length and gas tube length because both of these allow for more room for the gas to expand…as the gas is given more room to expand, the pressure within the system decreases. In short, you cannot affect volume or pressure of gas without also affecting the other; they are not independent.
Thats exactly what i was thinking…Their dependent upon one another. So, a smaller GP would delay the pressure build up which would result in less pressure throughout the time of pressurization when compared to a larger port???
There are more gas laws than just Mssr. Boyle’s.
Yes, port size does matter, otherwise goofs with drills couldn’t screw up otherwise properly-gassed rifles.
What the port size does, within limits, is act as a throttling aperture. a smaller port will delay pressurization, but the gas tube (the complete system) will reach full pressure, and quickly. The definition of work is “area under the curve.” The delay may not have as much effect on work as you might think.
The system is just that: a system. you have a balance of pressure, onset time, gas expansion, dwell time, etc. Yes, you can change one of them, and the others will adjust, but you’re just fiddling around on the margins.