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As you seat the bullets deeper you reduce the size of the powder chamber in the case, much the same as raising the compression ratio in an automotive engine. The higher the compression with the smaller chamber, the higher the pressure and the higher the velocity.

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Would someone explain to me how bullet seating depth changes affects the velocity of a bullet. thanks

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Hey MD, I guess ALL THE ABOVE cleared it up for you?!?!?! So,here is a bit more for an added confusion factor.

From my experience, you will see higher relative Pressure, then lower relative Pressure and then higher relative Pressure as you vary the Seating Depth using the exact same Load.

Lets say you start out doing your Load Development with the Bullet Seated so it is jamed 0.010" into the Lands. (More than this and the Bullet will normally slip farther into the case as you close the Bolt.) Work up your Load to a SAFE MAX watching for all the normal Pressure Indicators.

Then begin Seating the Bullet deeper into the case. As you get away from the Lands a few thousandths, the Pressure will "typically" go down a bit - for awhile.

Then you reach a point where the "Peak Pressure" begins to rise again due to the smaller "initial chamber" volume.

This is easier to see(happens quicker) in straight wall cases like a 444Mar, 45-70, 357Mag, 44Mag, etc., but the same thing applies to bottleneck cases as well.

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In the real world, I'm working with a new 338WSM now developing loads. As you seat the bullets deeper you reduce the size of the powder chamber in the case, much the same as raising the compression ratio in an automotive engine. The higher the compression with the smaller chamber, the higher the pressure and the higher the velocity. You get the same effect with heavier brass (less internal capacity.) In loads fired just this last Monday I'll give you the velocity and seating depths stated in the bolt-to-ogive measurement. The load was H4350-74g behind Nosler 210 Partition. You be the judge. The longest measurement, the ogive closest to the lands, was 2.520", 2916fps. At 2.510", 2921fps. At 2.500", 2925fps. At 2.490", 2931fps. The was over an Oehler 35P. THAT has been my experience consistently notwithstanding other "theories" expounded here in previous threads.

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How many rounds per seating depth did you fire to give you these averages?



From your figures, it appears obvious that there is no statistically significant difference in the velocities you obtainsed, since the shot-to-shot variation between IDENTICAL loads is usually greater than the mere 15 FPS you report! In short, your results do NOT prove that the miniscule reduction in case volume, and resultant increase in loading density one obtains by seating a bullet a little deeper, has any effect on chamber pressures or bullet velocity at all!! (NOTE: These conclusions pertain to RIFLE ammunition, NOT pistol cartridges that use much quicker powders!!)

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The way I do it is I draw the case on Quick-cad, the measure the bullet length and insert the bullet 3/16 into the case drawing and measure the overall length of the drawing.

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