Home Forums Hunting & Shooting Ask The Gunwriters seating depth vs powder charge

Just got back from a solo trip to the range. No one to talk to but my own thoughts.

When we are looking for the sweet spot when loading to get smallest groups, isn't changing to a shorter COAL doing the same thing as increasing charge slightly? I'm not saying there's an exact correlation but at some point the changes will be equivalent.

But I know from experience that they act differently, but why? Because rifle looneys like to chase their tales and make things more complex? Seems to me each are answering the same question: a which pressure does this rifle barrel vibrate just right to give the smallest groups.

Changing to a shorter COAL tends to reduce pressure in a rifle cartridge - gives the bullet a running jump at the throat Weatherby freebore style.

Increasing the charge slightly tends to increase pressure in a rifle cartridge.

About the only similarity I see is that changes in one factor change the overall result.

Increasing the COAL and seating closer to the lands tends to increase pressure so maybe there's a relationship there and maybe not.

You are OVER thinking things, have a shot of Brownstone and re-evaluate in the morning. !!!!

Decreasing COAL will progressively decrease velocity



So yes, at some point you will have the same effect on velocity that decreasing the powder charge would have. In the example above the decrease was 53 fps with a change in seating depth of .060". I could have gotten the same VELOCITY change with approx 1 grain of powder.

But IMO that would not have the same effects on accuracy, standard deviation or extreme spread. A certain bullet will prefer a certain jump because of multiple other factors

I understand the bit about the running start reducing pressures, but....

Is there EVER a point at which increasing compression of the powder results in pressure increasing?


Some software like the very interesting Powley Computer site (kwk.us/powley.html - I think the owner of the site is a member on here) suggests that shortening oal actually increases pressure.

By reducing oal are you not decreasing net case capacity?

Or is pressure and velocity potential more linked to the combined case capacity along with the capacity of the rest of the chamber all the way to the lands?

Sorry that I have posed the above questions in clumsy terms but I have always found the two concepts to be at odds with one another.

Yes there is a point at which pressure will go back up but that is past 1/4" or so



But at the typical depth range we seat bullets it is not a factor



The brass case has very little if any effect on the pressure except as it restricts the total volume of the combustion chamber. Your combustion chamber is the size of all your chamber minus the solid volume of the brass in the case. The solid volume of the bullet tail from the ogive where it hits the lands to it's base has the same effect as the case, i.e. takes up space and reduces combustion chamber.

Thanks Woods, have seen that graph before and it does help. But is it the distance from the lands that effects the pressures or the reduction of case capacity?

I know the two are linked of course, but which factor is it that is actually making the pressure rise after that approx .25 inch point?


Again I don't think I have expressed myself very well so I apologise if I sound like a dimwit!

IMO the case internal volume has very very little to do with it. We push brass around on the bench all the time with very little force. Inside the chamber when you're talking about 60,000 psi, well there is no comparison, like comparing a grain of sand to an acre of beach

Would just be guessing on why the pressure goes back up after .250". Might have something to do with the bullet hits the lands and does not engrave immediately, so there would be progressively more force needed to get it started again. For example if you had .500" jump then the bullet would jump to the lands, completely stop and then have to be restarted similar to seating against the lands to begin with.

Thanks for persevering with me Woods. Its a tricky subject for one of limited brain capacity like me.

I believe the jump to the lands has a far greater effect on pressure than the case volume. Maybe at that .25" mark, you could be getting bullets tilting and having trouble just entering the throat area straight?
The Lee loading manual has quite a bit of info on pressure vs seating depth in it if you have one handy.

Woods, I want to thank you for all the informative diagrams and research. It truly helps my understanding.

To maybe distil a little further of what I was thinking vs what you are saying... I was thinking the "pressure chamber" was the inside of the brass up to the base of the seated bullet. The lower the bullet was in the neck, the smaller the chamber and the higher the pressure for any given amount of powder given all other variables are equal... but that's wrong apparently.

So, the pressure chamber size is really determined when the bullet hits the lands, or how it hits the lands plus a multitude of other internal ballistics factors you need a phd to understand. My assumption is because the pressure to start a bullet out of the brass is insignificant as compared to what it takes to engrave the bullet into the rifling it makes little difference.

What I also infer from your answers is that full length sized brass vs neck sized brass should have very little effect on peak pressure because, again, the effort required to stretch the brass out to chamber size is insignificant as compared to pressures generated at peak.

Am I on the right track?

Yes.

Absolutely

Cute kid nic