cdoc42 said:
It's a question of which effect, the gas bypass or the restriction of powder space, is dominant. Below a certain seating depth around where the gap between the ogive and throat starts to be bigger than the gap between the case neck and the bullet, there is less additional effect of gas bypass on pressure. Seating still deeper, the effect on powder space is starting to have more effect. Dr. Lloyd Brownell had data for a curve for a round nose bullet that looks like the one below. It's a case in which throat contact only produces about 10% increase in pressure. That and the great seating depth length it takes to find the minimum are due to the fact the tapered portion of the RN ogive is a very gradual taper and requires a larger change in seating depth than a Spitzer shape does to open the gap around the nose a given amount.
cdoc42 said:
The trick here is to remember The Pressure most people refer to is the peak pressure. It occurs when the bullet is only an inch or two down the bore. Then pressure drops off due to expansion as the bullet goes down the tube. That falling pressure nonetheless is still high enough to contribute to further acceleration of the bullet. The bullet's final velocity then depends on the overall pressure it experiences in the tube. When you make more room for powder and put more powder in, even if you only put enough in to make the original peak pressure match, the fact you have a greater total quantity of powder means you make a greater total quantity of gas. That greater gas quantity means pressure after the peak doesn't fall off with expansion as quickly as it did in the original load, so the additional acceleration that occurs after the peak is greater than it was after the peak in the original load. That's where the added velocity comes from.
