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Jan 19, 2016

The ideal gas law also makes the assumption that the actual gas molecules have such a minuscule volume compared to the container that it can be effectively ignored. This holds for pressures around atmospheric where the mean distance between gas molecules is on the order of 30 angstroms (or about fifteen times the axial length of a Nitrogen diatom) but at very high pressures the molecules are much closer together and the amount of volume they occupy will be significant.

This is handled by subtracting a correction term (usually b) from the volume,
P1(V1-b) = P2(V2-b)

This won't influence the V2 portion of the equation much because the gas will have expanded to the point where the correction term is insignificant again but a lot of the initial chamber pressure probably comes from the fact that all of that powder that was taking up free volume in the cartridge is now a gas which takes up roughly the same amount of free volume in the cartridge (P1 is large because V1-b is small and nRT is constant). I wouldn't be surprised if the b term reduced the effective initial volume by as much as a half (and so increased the initial pressure by a factor of 2). Of course, this all gets a lot more complicated when you factor in burn rates and supersonic fluid mechanics, but the takeaway is that the final pressure will likely be considerably lower than you're predicting using the ideal gas law, meaning you'll need a larger gas port.

I'd suggest what others have, figure out some way of doing it by enlarging the hole incrementally from a best-guess starting point.

Edit: the sensitivity of chamber pressure to things like powder compression and bullet seating depth is strong evidence for the idea that b takes a significant chunk out of V1 in firearms.

Here are some PV curves for the ideal gas law (purple), b = V1/4 (orange) and b = V1/2 (green). The red vertical line is your unadjusted cartridge volume and the black line is your barrel volume at the gas port.

MrBusiness fucked around with this message at 20:45 on Dec 27, 2016


Jan 19, 2016

So I guess the hard part would be collecting data on the port sizes of AKs in different calibers and gas system lengths.

If you get this working, put me down for an AK kit.

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