lysanderxiii
Member
2700 K is the flame temperature of the burning propellant, and 5500 fps is the speed of sound for a gas at that temperature. Sorry, I didn't give you exact numbers, but I figured what's 20% between friends for a thumb-nail sketch of what's going on in an M4 gas system.1500 m/s is 4921 fps, which is below 5500fps. So you're saying that the speed of sound at temp is the issue, or it isn't? Is 4921 close enough to 5500 to mean that restricting the port below .06" than expanding back to .120 in the gas tube has no consequences for particulate condensing out of gas?
The primary thing I have been trying to get at is that the AR15 bolt and gas system was designed around a certain amount of flow and pressure that is found in the 20" version. Tapping the gas other places, using a smaller port, then reducing that diameter further is not going to duplicate the design parameters. And there has to be a point when a gas restriction is simply too small for good function and longevity.
Apparently, quite a bit for some. So, here is a map of the temperature.
If you take the temperature (in K) and plug it into the formula for the speed of sound at that temperature, you will find the entire gas flow through the port is sonic except in the stagnation points.
As to condensates, the temperature of the gas in the tube stays above 1000 K until long after the bullet is gone and the pressure has dropped. The temperature of any gas system shorter than a rifle will be higher than a rifle, so unless you are using that crappy propellant they used back in 1966 that caused all those problem, you won't have a problem.
[To the bold] Yes, you can. I provided links to reports that have the necessary equations and information to model the system. The only major obstacle is the range of port pressures that work for shorter gas systems gets smaller as you shorten the tube length.
A 16" barrel with a mid-length with a .078 port and a standard buffer is very close to a rifle length with a 20" barrel and rifle buffer, as far as bolt velocity goes, over the same range of port pressures. And, that range is much wider than a 16" with a carbine length gas system, or even a 14.5 with a CLGS.