Pressure changes with seating depth

What have you found in your reloading experience? Does seating a bullet deeper in the case raise or lower pressure?

let's say charge weight stayed the same, but you went from being up on the lands to a .050" jump. Have you found this to change pressure much?

I raised that question in a different thread and never got any feedback. I assume you are not getting a compressed load with the deeper seating.

Capacity is pretty full, not much wiggle room where it's sitting now. There's an upper powder node a mile wide, but on the top end of it I'm riding some pressure signs. I have yet to tune seating depth and an afraid of seating them deeper will affect pressure negatively.

I've heard that loading deeper increases pressure with the same charge because the bullet is eating up case volume. I've heard that loading long increases pressure because the bullet doesn't have any momentum behind it when the rifling begins to engrave it. Honestly, I have no idea. They both kinda make sense to me.

If you're not already near max I'd think you're probably safe playing with seating depth.

I dont know about pressure other than jammed is higher, but i did load development at .07 off the lands for my 7mm blaser and eol195s. I went up to pressure and had a node comfortably under any pressure. Then i used that charge and seated bullets out all the way to .01 off. From .07 to .01, i gained 44 fps. The h180s gained 31 fps on the same seating change.

I have measured relatively subtle differences in velocity (pressure) with seating depth. Someone with QuickLoad can probably give a more thorough answer.

As you seat longer, you lose velocity. PV=nRT. Since n, R, and T are mostly fixed for a given charge weight, then if V goes up (less bullet in the case), P (velocity) goes down.

That being said, once you start getting pretty close to the lands (0.02" off?), you are now entering the "jam" realm. Depending on how the pressure curve is building in the case, if the bullet is suddenly stopped, pressure can rise rapidly while the bullet overcomes this.

I personally would not seat a bullet longer if I was already near both the lands and my max pressure. Seating it a little deeper probably won't mean much difference. It will unquestionably raise the pressure, but in my experience with 300 WM and 270 Win, 0.05" shorter probably didn't even equal 0.1 gr more powder. For example, 0.1" seating depth in my 300 WM was 10-15 fps, and only obvious with lots of shots and long term averages.

entoptics said:

I have measured relatively subtle differences in velocity (pressure) with seating depth. Someone with QuickLoad can probably give a more thorough answer.

As you seat longer, you lose velocity. PV=nRT. Since n, R, and T are mostly fixed for a given charge weight, then if V goes up (less bullet in the case), P (velocity) goes down.

That being said, once you start getting pretty close to the lands (0.02" off?), you are now entering the "jam" realm. Depending on how the pressure curve is building in the case, if the bullet is suddenly stopped, pressure can rise rapidly while the bullet overcomes this.

I personally would not seat a bullet longer if I was already near both the lands and my max pressure. Seating it a little deeper probably won't mean much difference. It will unquestionably raise the pressure, but in my experience with 300 WM and 270 Win, 0.05" shorter probably didn't even equal 0.1 gr more powder. For example, 0.1" seating depth in my 300 WM was 10-15 fps, and only obvious with lots of shots and long term averages.

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Does powder burn rate figure into all this pressure equation. Do the slower burning powders (4831) need more unfilled case vs. 4064 at max case capacity with seating depth determined by space for powder to burn.

Nosler has some good Load Density/volume on there on-line load data info. Would give you some idea of how much room you have for how deep you could seat bullet below neck.

shooter7 said:

I've heard that loading deeper increases pressure with the same charge because the bullet is eating up case volume. I've heard that loading long increases pressure because the bullet doesn't have any momentum behind it when the rifling begins to engrave it. Honestly, I have no idea. They both kinda make sense to me.

If you're not already near max I'd think you're probably safe playing with seating depth.

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This.....Im trying to find the article but someone did a pressure test. They found that when the bullet is in the lands, there is a significant jump in pressure. Then when you start backing out of the lands, the pressure falls but at a certain point starts to increase again as you seat the bullet deeper into the case especially if the case is relatively full with powder. I think depth ranges from .010 off the lands to .12 off had very similar pressures with the same powder charge but I honestly cant remember. As with any reloading, when you start going toward extremes you need to be super careful.

Quicksilver338 said:

What have you found in your reloading experience?

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I do seating testing (Berger's recommended) before testing powder. This is at a charge predicted reasonably lower than max, giving me plenty of room pressure-wise. Don't go to a lower node (as expected) though. You need to stay away from any powder nodes for seating testing (so you can see seating results).

You could do the same now, and then you might go back to powder testing with your new best coarse seating. After powder, tweak your seating within it's window for tightest group shaping.

entoptics said:

I have measured relatively subtle differences in velocity (pressure) with seating depth. Someone with QuickLoad can probably give a more thorough answer.

As you seat longer, you lose velocity. PV=nRT. Since n, R, and T are mostly fixed for a given charge weight, then if V goes up (less bullet in the case), P (velocity) goes down.

That being said, once you start getting pretty close to the lands (0.02" off?), you are now entering the "jam" realm. Depending on how the pressure curve is building in the case, if the bullet is suddenly stopped, pressure can rise rapidly while the bullet overcomes this.

I personally would not seat a bullet longer if I was already near both the lands and my max pressure. Seating it a little deeper probably won't mean much difference. It will unquestionably raise the pressure, but in my experience with 300 WM and 270 Win, 0.05" shorter probably didn't even equal 0.1 gr more powder. For example, 0.1" seating depth in my 300 WM was 10-15 fps, and only obvious with lots of shots and long term averages.

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What method did you use to test the pressure? Special tools or look at primers, swipe, etc? Thanks

Push the bullet back far enough into the case and it will raise pressure because of the now smaller case capacity that has been created. I'm speaking with reference to going by the reloading manual and starting with a SAAMI seating depth, which I personally never do. I usually start at .010" off the LaG's with every bullet I try, no matter the ogive.
Although I will say I recently received a valuable lesson with seating depth(s) while trying to find a sweet spot for the Berger 95gn VLD in my 6BR Norma. I tried various depths and then called Berger, which they suggested I try it jammed but at charges closer to starting loads, as some of their techs had discovered this. I reduced powder levels to starting loads and tried the jam. That worked great and I found a superb load within a few shots. That's just that bullet though and not all Bergers or rifles will react this way.
As you slowly work back from/off the LaG's pressure will decrease until you reach a certain spot or depth. Then you're approaching "SAAMI" depth's and in some rifles that may be magazine depth or with some bullets it may be lightly compressed.

I read an article a while back about seating depth for revolver cartridges, wherein the author was using SAAMI depth reloads (read as pressure) and then seated the bullet out farther, up to his cylinder's length. His findings were once the bullet was seated out deeper, pressure went down enough so that more powder could be added to increase velocity and still be within the pressure limits established by SAAMI.
This made me think of Weatherby cartridges, which are loaded to their established SAAMI length/pressures. The chambers, however, have a bit of freebore, which causes the bullet to pick up speed before it encounters the LaG's of the barrel. This concept seems to work well.
I recently rebarreled my .300WinMag. The 26", factory barrel with SAAMI cut chamber shot really good and was what I call a "fast" barrel. The primers always seemed to be cratered though, no matter the load, from mid to top loads tried. I never worried about it because no other pressure signs were apparent.
I assume this was because of a tight chamber. Finding the seating depth for reloads with various bullets was a little depressing for me. I think a cartridge just looks better with a bullet run out farther than the "book".
I replaced that shot out barrel with a 8t, 28" Pac-Nor, which I also ran a Ryan Mantha finishing reamer through before installing. I think this reamer was created so that the 215gn Bergers could be loaded with a COAL in the 3.7" range, as it did for me. Accuracy and speed are still there but no cratered primers and no other high pressure signs. It's the same charge as the old barrel liked but feels milder in the P-N
barrel. The benefits of this chamber are not only apparent with Bergers but also with the 212gn ELD-X's. Their ogive is not the same as the Berger (more tangent that secant, kind of a hybrid look) but the velocity and accuracy are almost identical but the ES & SD are a little lower for the Hornady's. Go figure.
In summary; you have to find that ideal seating depth for pressure and accuracy. Too much in either direction (into the lands or into the case) will cause excessive pressure, relative to your powder charge.
This is long but I hope it helps.

I think your scenario is actually combining two issues at the same time; seating depth and jump/jam. If you separate the two issues it will be a little easier to answer the question. Seating the bullet to a longer cartridge length will lower the pressure, to a point. Seating a bullet into the Lands will raise the pressure, but may improve you accuracy with certain bullets. But in your case if pressure is already near max, and if you are .050" off the lands and then try to seat to the lands, you are probably going to see a spike on pressure. Now if you were .150" off the lands and seated .050" longer, it would decrease your pressure because this would give you more relative case capacity. But when that same .050" increase puts your bullet into contact with the Lands, your pressure will increase. That's why SAAMI Standards were established, to set factory standards that would keep commercial ammo safe.
Now, that being said, the trend lately has been to use heavy for caliber bullets, seated longer than SAAMI specs, in chambers with throats cut longer to accept those long bullets and fast twist rates to spin and stabilize those long bullets. Where we get in trouble is when we try to use those long bullets in a standard chamber. To make them fit in the chamber, or magazine , we seat the bullet deeper into the powder space in the case.
I know I'm kind of rambling here, but what I'm trying to say is your weapon and ammo must be designed to all work together. If you're going to shoot something like the new Hornady 153 gr. A Tip bullet, you have to have a rifle designed for it.

Mram10us said:

What method did you use to test the pressure? Special tools or look at primers, swipe, etc? Thanks

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Good ol' Chrony. All things equal (powder, primer, bullet, case, rifle) then V = P for all intents and purposes.