Load Density, Bullet Jump, Case Air Space

Highlander44

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I know load density and bullet jump has been beaten to death but one issue I cannot resolve completely is how the resulting air space affects burn rate and pressure when a bullet is set against the lands. Or shorter such as SAAMI COAL. The load density is the same but air space can be different which intrigues me. Not sure why but one of those questions you ask yourself eating a Twizzler.

JE Custom once stated the extra oxygen can affect burn rate and that statement has "burned" in my memory for a while and recent loading of the .270TH has me thinking about it again once more. JE also stated bench guys preferred compressed loads for that reason to have consistent burn rates.

Sooo, my far to much time to think mode has me wondering the bullet should be seated at juncture of the powder column of slightly compress instead of trying to seat out against lands. I am shooting the 156HH and I may try some measurements to seat deeper in case and run ladder against previous results of stretching them out to max COAL.

Try this or go back to Twizzlers?
If you have an air space in the case, your primer may be igniting a different surface area of powder between shots. We shoot angles in the mountains a bunch, and have noticed this when shooting downhill. We were hitting high even after compensating for the angle. We figured we were igniting a larger surface area than when we were shooting flat.
because of this, now we always try to find an accuracy node and powder that doesn’t leave much room in the case.
 

orkan

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now we always try to find an accuracy node and powder that doesn’t leave much room in the case.
You get all kinds of other benefits from this as well. Typically the powders that fill the case have a bit slower burn characteristic, which produces a more gentle and desirable pressure curve. This gives better barrel life and much more forgiving nodes which can withstand drastic changes in atmospherics and bore condition and still hold together. All around a very stable way to be. Big reason why I run H4350 in my BR-based stuff rather than varget.
 

MagnumManiac

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When I mentioned packing scheme of powder, I was actually talking about a full case that is slightly compressed or properly compressed.
If you dump powder straight from a measure, or swirl charge the powder into the case with a funnel, there will be a large difference in powder height.
As dumped charges are ‘fluffy’, you will see a significant difference in powder height across a full loading block. Swirling the charge leaves the powder height very uniform, you will not see any change in powder height and even though the 2 charges are the same in the case, the velocity will also be different and more uniform. A properly swirled charge will not settle even further even when vibrated.
A powder that settles through travel or handling may have some rounds in the node and others falling outside of it.
This is the same as those experiencing a difference in drop when shooting downhill compared to uphill. The air gap is altering the burn/ignition.
Factory ammo is crimped for a reason, it alters start pressure and they know they can utilise this when using a powder that doesn’t fill the case. Believe it or not, if you run most factory ammo over a chrono they do run pretty uniform for such a large air gap left in the case. The difference without the crimp would be night and day.

Cheers.
 

Mikecr

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The bullet seating depth does not change the load density, only the effects of seating depth to powder and pressure.
You are absolutely wrong.
Seating depth changes load density as it changes effective case capacity per given charge.

And best seating is not determined by load density.
It is an abstract that can only be found through full seating testing, regardless of powder/charge.
In other words, seating results(for best accuracy) are independent of powder, capacity, or load density, even while capacity and load density are directly affected by seating.
 

longrangebob

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No, the load density is not the same.
With lower load densities the burn becomes a bit more abstract, which affects results.

If you would like to directly observe this:
-Load 2 rounds at ~85% load density
-Load one with the gun pointed down, slowly raise to level, and fire at target or across a chronograph.
-Load second with the gun pointed up, slowly lower to level, and fire.
-Note the MV or POI difference.
You might also cause the same affect based on how fast/hard or slow/easy you chamber your rounds.

A high load density is not sensitive to these things.
Partly why I prefer as slow a Powder as “Proper” for a Cartridge so as to achieve a full Case 😉
 

MagnumManiac

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To add to what Mikecr just said, full seating testing is not just seating depth testing per se, it is also how much of the neck is sized, as this also affects seating depth testing and bullet release.
I discovered this recently again when I sized down 7RM to 264WM. I sized the necks just enough so they would chamber with a little pressure on bolt close for a better fit. When bullet seating, the heel of a Nosler 140g Accubond is below the neck/shoulder juncture and into the pressure ring normally. There was no pressure ring present and these loads were printing into a ragged hole….could I get the same result after firing and bringing the necks back 90%….not a chance, that slight increase in diameter at the neck juncture was the only difference in those parameters. The .004” difference in neck juncture diameter wasn’t enough to keep them in one hole.
I am going back to this, as I haven’t fireformed all the brass yet, still testing primers and I have to say, this batch of Remington brass is very good in volume as is my latest batch of 300WM and 7STW which I have 100 in each, and the now 264WM converted brass.
I am pleased with it.
Just gotta get out there after this last lockdown.

Cheers.
 

Mikecr

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Overall, I say go with a fastest powder -that fills the case -at SAAMI max pressures.
Your tested best powder load will be near there.
Of the fastest powders meeting this, go with the coolest burning for better barrel life.
QuickLoad makes all of this easy.
 

Eeegadd

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I know load density and bullet jump has been beaten to death but one issue I cannot resolve completely is how the resulting air space affects burn rate and pressure when a bullet is set against the lands. Or shorter such as SAAMI COAL. The load density is the same but air space can be different which intrigues me. Not sure why but one of those questions you ask yourself eating a Twizzler.

JE Custom once stated the extra oxygen can affect burn rate and that statement has "burned" in my memory for a while and recent loading of the .270TH has me thinking about it again once more. JE also stated bench guys preferred compressed loads for that reason to have consistent burn rates.

Sooo, my far to much time to think mode has me wondering the bullet should be seated at juncture of the powder column of slightly compress instead of trying to seat out against lands. I am shooting the 156HH and I may try some measurements to seat deeper in case and run ladder against previous results of stretching them out to max COAL.

Try this or go back to Twizzlers?
Few bullets like being seated on the lands. Try .020 off 3 to 5 shots each at .003 increments. Look for 2 groups close together. Then do your ladder test again. It will change. Then you can forget about the above and just shoot dimes.
 

Muddyboots

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You are right on load density IF my bullet is ACTUALLY touching the powder column. My BAD for not emphasizing my COAL is likely not allowing bullet base to touch or compress the powder column. Again MY BAD for being unclear in my thoughts. My desire is just touch and not compress the powder column thus eliminating air space and in theory improve ignition.
 

Frogman77

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This was mentioned before but different powders behave differently. Some like compression, some do not and some of it is cartridge dependent.

h1000 in my wsm does really well with lots of compression. I can’t even get enough in the case to hit pressure signs.

Retumbo in my 338 norma goes from easy shooting to locked bolt with just a little compression even though based on quickload I should be well under saami max pressure.

In general I try for loads that are slightly compressed that don’t generate more than 63kpsi based on quickload predictions. I know quickload isn’t Completely accurate when it comes to actual pressure, especially with solids. But I’ll usually confirm with simple pressure testing ladders, and almost always when using jacketed Bullets when I get close to 64kpsi Based off quickload I start seeing the first signs of pressure. Anything behind that I steer clear of.


Bottom line, when tuning loads there are a lot of variables. Does Load density matter? In my opinion it does. I try to keep my case fill somewhere between 100% and 106% if the case allows for it with out crushing Bullet jackets when seating. I have gone higher but you need to use a long drop tube or vibrate. The cases to settle the powder.
 

asd9055

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You are right on load density IF my bullet is ACTUALLY touching the powder column. My BAD for not emphasizing my COAL is likely not allowing bullet base to touch or compress the powder column. Again MY BAD for being unclear in my thoughts. My desire is just touch and not compress the powder column thus eliminating air space and in theory improve ignition.
My understanding of load density is the ratio of the volume of the powder over available Volume. As you move the bullet up or down in the case, the a available volume changes, without changing the volume occupied by the powder, thus load density changes. I could be wrong, but that’s my understanding
 
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Muddyboots

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Only if the bullet compresses the powder, the powder volume is not changed it still occupies the same volume of total case volume.
 
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