Whats spikes pressure more?

[KYbulldog]

Physics and reloading question...

What would cause a net increase in pressure more...reducing case volume or OAL of cartridge being such that the bullet is at Lands?

3 cartridges....
Take a 175 grain smk loaded to 2.80" (the control)
now seat the bullet(with the same powder charge) to an OAL 2.75"(or something less than 2.80) thereby reducing case volume.
now take another(same charge) and load to an OAL such that the bullet is now just seated into the lands.

Which of these latter two would see a higher pressure as compared to the original load if any?

 

[6/250/40]

To the lands.

 

[atlsniper6.5]

I am interested to hear what people have to say about this. Just to provide some data though... I recently started load development on my 260 Rem with new Lapua brass and 140 hybrids seated 4/1000 off the lands. I went back out after fire forming (neck sized only) the brass, and I seated the bullet back to 10/1000 and I lost about 80 fps. Not sure if the velocity loss was from the brass expanding or the bullet being seated deeper.

 

[leadbullet]

both

 

[milo 2.0]

I am interested to hear what people have to say about this. Just to provide some data though... I recently started load development on my 260 Rem with new Lapua brass and 140 hybrids seated 4/1000 off the lands. I went back out after fire forming (neck sized only) the brass, and I seated the bullet back to 10/1000 and I lost about 80 fps. Not sure if the velocity loss was from the brass expanding or the bullet being seated deeper.

Did anything else change too? Accuracy? You should be able to get the speed back by increasing powder charge.

I'd say jamming into the lands will create the highest pressure.

 

[atlsniper6.5]

Did anything else change too? Accuracy? You should be able to get the speed back by increasing powder charge.

I'd say jamming into the lands will create the highest pressure.

No, it was still very accurate.


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[KYbulldog]

Did anything else change too? Accuracy? You should be able to get the speed back by increasing powder charge.

I'd say jamming into the lands will create the highest pressure.

Not letting fps factor... Lets add the fact that you are already at max load pressure wise, not max load because of compressed charge, but pressure signs (popped primer, hard extraction, ejector swipes) …so there is still ample case volume.

I am interested to hear what people have to say about this. Just to provide some data though... I recently started load development on my 260 Rem with new Lapua brass and 140 hybrids seated 4/1000 off the lands. I went back out after fire forming (neck sized only) the brass, and I seated the bullet back to 10/1000 and I lost about 80 fps. Not sure if the velocity loss was from the brass expanding or the bullet being seated deeper.

From what I've read, fire forming will increase case volume depending on chamber, thereby reducing pressure. Seating the bullet back farther, and reducing case volume, should have the opposite effect of increasing the pressure. But at 80fps I'd say not by equal amounts.

I'm sure this is way oversimplification.

 

[atlsniper6.5]

Whats spikes pressure more?

Not letting fps factor... Lets add the fact that you are already at max load pressure wise, not max load because of compressed charge, but pressure signs (popped primer, hard extraction, ejector swipes) …so there is still ample case volume.




From what I've read, fire forming will increase case volume depending on chamber, thereby reducing pressure. Seating the bullet back farther, and reducing case volume, should have the opposite effect of increasing the pressure. But at 80fps I'd say not by equal amounts.

I'm sure this is way oversimplification.

Your logic is correct, but back to the original question... How much did the bullet being jammed in to the lands increase the pressure?

I guess a good secondary question would be, what is considered jammed? I would consider 4/1000 to be jammed, which would explain my loss in velocity despite the fact that I seated the bullet deeper (not jammed). If I'm wrong the velocity loss is strictly from fire forming.



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[MarinePMI]

Net increase? None. Pressure spike at initial ignition? Seated in the lands...

 

[J-ROD]

I seat my bullets long for ftr, if I jump too much or jam with my hot load, I get similar pressure signs on my brass, fps stay in the same ballpark.
Fwiw, thanks to the help of McCourt munitions, I just took a load that was multiple moa and made 1/2 moa vertical wise by going from .010" jump to .030" jump. Same everything else. Food for thought

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[KYbulldog]

Not so fast..not so simple
Just going from imr online reload data site comparing two bullets loaded to same COL. The 168 sie hpbt and 168 Barnes TTsx loaded with xbr8208...one would think the longer bullet (loaded to same length creating less volume in case) would produce higher pressure. But that's not what the data shows, assuming its correct. The longer Barnes bullet shows less pressure. Blows my theory. And from what I read elsewhere it's been confirmed that a bullet at the lands produces more pressure than reducing case volume.

 

[pawprint2]

As the two loads in question are not loaded with the same powder, I do not think one to one comparison has been achieved. Other than the reduction in case volume due to the length of the projectile, the jump to engraving would only be the same if the two projectiles had identical profiles. Usually "jam" seating creates the higher pressure, as an object in motion tends to stay in motion, etc. it is harder to get the projectile, a body at rest, moving. This small delay causes for a more vertical spike, due to decrease in volume as compared to a bullet leaving the case sooner, as volume increases, pressure decreases. The only sure test is same bullets/cases/primers/powder from a pressure gun. My guess is, you'll almost always have a higher pressure with "jam" seating.

 

[Fred_C_Dobbs]

The property you're alluding to is called "shot start pressure" or "shot initiation pressure."

The answer to your question is here, about 4/5ths of the way down the page, in a sentence that starts, "To illustrate the effects of variations in bullet travel before the bullet enters the rifling,..."

However, unless your question is purely academic, I doubt that Hornady's answer will satisfy you, because shot start pressure does not control, nor is it directly tied to Pmax or MV. Higher (or lower) shot start pressure does not guarantee of higher (or lower) Pmax or MV.

 

[Rprecision]

I think your confusing case capacity with seating depth, the two are not the same.

Take two .308 cases, a Winchester and a Lake City. The later has less case capacity because it has more brass and less room for powder.

If you take a "normal" load lets call it factory 168 FGMM load. Seat it a additional .100" you didn't reduce case capacity, you seated the bullet deeper. The bullet is the moving part. Case capacity is set when the pressures peak at the throat.

 

[TacticalBoltKnob]

If you take a "normal" load lets call it factory 168 FGMM load. Seat it a additional .100" you didn't reduce case capacity, you seated the bullet deeper.

The unloaded case, yes. The loaded cartridge, no.

The deeper you seat that bullet, the less unused space there is in that case and the higher initial pressure will be when that powder ignites. Which is why if you set COAL so that the bullet is touching or jammed into the lands, the initial pressure spike is even higher, because there is resistance to that initial pressure.

 

[Fatelvis]

Quickload says to add 7200psi to the pressure figure when seating to the lands. That seems like a considerable jump to me, especially if the load is close to max already.

 

[40xs]

It's all relative and dependant on to what degree of each factor: minor vs. major. One just can't say that "this" causes more pressure than "that". A little of "this" wouldn't be as bad as a lot of "that". you could turn it all around by simply adding to or taking away from the other factor. In the end, deviating from perfection in all areas on the same cartridge could be catastrofic.