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Originally Posted by MarkCO
You would be better off looking at choked flows and supersonic flow theory. There you would gain an understanding of those who say that the form factor (bottlenecked case) does not result in a pressure increase cannot grasp. In very simple terms, you have water flowing from a hose at full diameter. If you restrict the opening through which the water is flowing...what happens?
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If the case was open to the air and the powder evolved gas fast enough or if you had a practically unlimited supply of gas, as your garden hose has with water, you would develop more pressure in the more confining smaller-necked case of two chamberings sharing the same parent cartridge (say, 30-06 and 25-06). The problem is the gun is not open to the air when the bullet is in it propelled. The bullet winds up being the gas flow rate limiter, especially in the first couple of inches of bullet travel when the pressure peaks. We know this because the muzzle blast spheres in shadowgraphs get out ahead of the bullet, initially, indicating the gas pressure allowed for greater gas speed than the bullet allowed it to have. Also, if the bullet were not the dominant factor in limiting that flow rate, having a longer barrel would not increase velocity as there would be no significant pressure behind it to accelerate it further.
You may find a small pressure difference due to case mouth narrowing by the time the bullet nears the muzzle, but the difference is just a few percent. I think QuickLOAD's model does a pretty good job of predicting this. It gives you both muzzle pressure and the pressure in the chamber at the time the bullet base gets to the muzzle. For a 100 grain bullet in the 25-06 fired with Varget to a peak pressure of 60,000 psi, with a 24" barrel the pressure at the muzzle is 21.5% lower than at the chamber. For a 30-06 with a 144-grain bullet (to match sectional density) the difference is 17.7%. So you get about 3.8% lower muzzle pressure due to the narrower bore. At the peak pressure, when the bullet is still moving slowly, the difference is going to be much less. On average you might see a 2% difference in bullet acceleration due to it. Not much.