[dgang]

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.

Unclenick: Just out of curiosity, in your comparison of a 25-06 and a 30-06 load what would the optimum barrel length be?