Seating Deeper Reduces Pressure? April 2019 Handloader

Brian Pierce writes this paragraph on page 15 in response to a letter about a 6.5-300 Weatherby:

Usually, but not always, seating a bullet deeper in large capacity bottleneck rifle cartridges will reduce pressures slightly so my deep-seated loads should not be of concern in this application

This is contrary to everything I thought about pressure vs volume. Seat the bullet deeper and you decrease the internal volume the powder has to burn in which increases pressure. Seat the bullet deep enough and you begin compressing the charge which can increase the pressure spikes.

Is he making a reference to slow burning powders only? Decreasing the volume would cause an incomplete burn?

Paging Larry Gibson. I’d like to see some actual pressure results before I believe this. Not saying it can’t be true. Just that w/ no pressure data it’s conjecture.

This is a new one on me????

In some cases it is true. Large volume cases are not effected as much by deeper seating. Deeper seating does increase pressure, however, more distance to the bullet engaging the lands reduces pressure. That is why Weatherby used such long throat. Hard/soft seating bullets into the lands will normally be the highest pressures in a cartridge like the 6.5-300 Weatherby.

The best example I have is a 308 Palma loaded I used. I soft seated the bullets on these and they shot outstanding until the temps got above 90 degrees than they started having pressure issues. This load also shot second best with .035" jump. Same load seated .035" didn't have any pressure issues above 90 degrees.

On the other end of the spectrum I have seen 9mm cases blown from bullets that deep seated due to recoil.

Think about the percent reduction in capacity. .100 deeper in a 9mm could be a 25% reduction in capacity. The same in that 6.5mm neck is maybe 1%. Neligable vs the decrease gained by adding to the effective freebore.

seating deeper in an actual weatherby chamber probably wont do squat as weatherby's have long throats to start with.

With large volume cases and slow powder there really wouldn't be a case/powder/bullet issue as many of them are/can be compressed loads. It would even help with to far of a reduction in powder charge. Thus seating the bullet deeper you get a longer jump before the bullet starts into the lands actually reducing pressure.

With small volume cases and fast powder like the 9mm, 45 acp, 40 s&w, etc it seems or I have read that it can increase pressure, even dramatically. Those cartridges headspace on the mouth and I am not sure of the different manufacturers lead from model to model. But, with general bullet types and weights with barrels ranging from 2 to 6" it would be good to get more knowledgeable input from those scenarios to see if the lead plays a role in the increase of pressure.

Seating a bullet 0.005" off of the lands results in less pressure than if the bullet is jammed into the lands.

Quote:

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Originally Posted by MT Gianni

Seating a bullet 0.005" off of the lands results in less pressure than if the bullet is jammed into the lands.

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^That! It's more accurate to say seating deeper affects the pressure curve. Seating longer will reduce pressure unless you seat into the lands. In that case, the bullet has to overcome it's own inertia + neck tension + the leade. Lead boolits are more forgiving but you get a spike with either