Author: **boilingleadbath** » Mon Apr 16, 2007 2:33 pm

My understanding is that a 4x hybrid generally gives you around 400 PSI, so (applying 1/(B:C ratio + 1)*400) we get about 226 PSI (final).

That's inaccurate (how inaccurate? I'm too lazy to check), but it gives you an idea... that the average pressure is probably pretty close to the average pressure you'd need to get 2700 fps out of this gun, as calculated by Joanna.

However, Joanna fails to acknowledge the rather massive amount of gasses involved in this:

80 grams of air originally in the barrel

430 grams of fuel-air mixture originally in the chamber

...and these need to be propelled too.

Now, of course, the 80 grams of air was expelled at an average speed of about 1/2 of the projectile's, so we only count half of it.

Furthermore, 60% of the former fuel-air mixture is in the chamber when the projectile exits, which we count it as 1/18th it's mass because of it's lower velocity, and the other 40% is moving at something like .7 (average) times the velocity of the projectile, so we only count 70% of that.

Add this up, and we get 170 effective grams, or so, of gas... which is not ignorable.