Gene,
I am glad we can agree that the change in weight of the air in the barrel is insignificant. (For the record, the air in the 22" 0.308" diameter barrel at 20C and one atmosphere of pressure weighs 0.498 grains, but the CHANGE in weight with temperature, altitude, etc. is much less.)
OK, so let's look at the force required to push the air out the barrel. There is about 50,000 psi pushing on the base of the bullet, which is in turn pushing on the air in the barrel. What these two must push against is the air in the atmosphere, which under standard conditions is pushing back with a pressure of 14.7 psi. The driving pressure is thus 50,000 - 14.7 = 49,985.3 psi. Now let's decrease atmospheric pressure by 2 psi (a large amount, by atmospheric standards). This increases the driving pressure to 49,987.3 psi, a 0.004% increase that would result in a change in muzzle velocity from 3050 to 3050.1 (from post #63). Not large enough to explain anything.
So what could the important effect be? Gene, as you asked me to, please open your mind and consider this: The thermal capacity of an 80 oz stainless steel barrel is 1135 joules per degree Kelvin. Compare this to the amount of energy that we get out of the gunpowder, for the 30BR, about 2200 ft lb or 2983 joules. So the barrel, by soaking up enough heat to raise its temperature one degree Kelvin quenches over 1/3 as much energy as we get out of the charge!
That is a huge effect.
Now how can the amount of heat that the barrel quenches change? By changing in temperature! Gunpowder burns at about 800 K, and a barrel at 80 F = 300 K gives a temperature difference of 500 K. Convective and conductive heat transfer is proportional to temperature difference, so a 10 K increase in temperature will cause a 2% decrease in heat quenched. Less heat quenched means more energy to drive the bullet to higher velocity. By the proportions above, a 0.76% increase in muzzle velocity from 3050 fps to 3073. Now that is enough to affect tune!
Granted, these are back-of-the-envelope (literally!) calculations, but they suggest that barrel temperature could be the important factor that changes tune. Jerry's experiments also suggest that it is temperature and not atmospheric density. Denton Bramwell's experiments show the same thing.
What do you think?
Cheers,
Keith
Jerry,
Your idea about the drag of fouling increasing with humidity is interesting. Maybe it could be tested by measuring muzzle velocity from clean and fouled barrels in both low and high humidity, at the same temperature. Have you already done this?
