Case Capacity, Load Density, Powder Charge

[dega37]

Would some of the experience guys on here please help me understand the topic of Case Capacity, Load Density, Powder Charge. When Case Capacity in grains of H2O is mentioned, does that mean filled from a closed primer port to the very top of the mouth with no bulge in meniscus? So if we were to say that filling the case to the top mouth just like water lets say was 32grains of powder is that 100% case capacity and Case Density? If we were to take that 32 grains at 100% then multiply that by .8 to get 25.6 grains is that 80% capacity?

For powder charge, published load data will say that a particular powder has a Max of lets say is 28 grains and a min of 26, that is related to pressure ( and pressure is related to powder burn rate, case capacity and bullet seat depth and weight?) How is that 28 grain max related to case capacity?

For example, Nosler publishes the custom comp 123gr hpbt with 2.250" in a 24" barrel. using IMR 4895 they list Max charge at 28gr and min at 26gr but then load density of max charge is at 103% and at min charge is at 95%. Based on that this makes me believe that this is actually based on bullet pressed into the case + powder and 103% load density means they compressed the powder slightly.

https://load-data.nosler.com/load-data/65-grendel/

Hopefully my questions make sense, please let me know otherwise and I'll edit.

[Lemonaid]

I'll try giving a practical answer as I understand it, expansion or corrections welcome.
Case capacity is mostly used to compare volume of different caliber cases (6.5 creed vs 6.5 Grendel vs .223 rem). It is the absolute maximum that can be filled into a case as a liquid with no bullet present. No powder could be compressed enough to get there. It can also give a indication between different brass makers (Lapu vs. Hornady) of the same cases, one may have slightly more room. Military cases being made extra strong will have enough diminished case capacity that the reloading manuals say start loads at 5% or so below commercial brass. Mostly ignore case capacity.
Load density is the percentage of powder under the bullet, over 100% and it is being compressed, the bullet is touching the powder. Seating depth of the bullet will increase or decrease this number. If it is under a minimum percentage (65?) it can be dangerous, chamber ringing or worse. Reloaders want to be in the 90%+ percent range if possible. Many prefer no compression.
If looking at a reloading site like Nosler's and you see a combination of powder and bullet that has a max number for powder and it has say 95 percent load density that means there is room for more powder but chamber pressure maxed out there so they stopped.
If the load density was over 100% and it had a comparatively low p.s.i. they maxed out on case space before pressure.

[Klem]

dega,

Can I add that any cartridge over 103% load density will be difficult to fit more powder into without settling the powder using drop tubes or vibrating the base of the case against your tumbler. Even then you get that uncomfortable crunching feel as you seat the bullet. Leave overloaded cases in storage for any length of time and unless they are crimped they will likely creep forward.

There is a linear load density/pressure relationship up until 100% full and then it rises more steeply as you compress powder. For faster powders such as 8208 in the Grendel range this is even more pronounced so you need to be careful that small increments of [compressed] powder have relatively large differences in results.

You mention meniscus when filling cases with water to determine volume by weight of water. It is a non-issue whether the meniscus is concave or convex. The difference between the two is not enough powder to make a meaningful difference in either pressure or results on the target. You want to be careful however that droplets are not clinging to the outside of the case as you weigh them.

Powder limits in load manuals are based on max SAAMI pressure, which in turn is based on the most fragile likely action in the marketplace. AR15's would be about the most fragile Grendel action on the planet so these guns are the canaries in the coalmine for over-pressure. If you are using an AR15 be mindful to pay attention to max loads, which tend to be conservative to avoid litigation and reputational damage. Even with an AR15 you can go slightly over manual limits but know you will be hammering your gun to destruction. It may take 1K rounds for something to wear or break but then again that might be totally acceptable to you, and the extra 50fps worth it.

You can go below 95% case capacity if you want, to save powder, or the gun, or for pleasant shooting, kids etc. Just don't go below 80% available case capacity (after bullet seating). Below 80% and you risk two dangerous outcomes: Flash-over and Secondary Explosion Effect (SEE).

[Fess]

Over the years I have seen some posts stating that compressed loads are sometimes less consistent that uncompressed ones. The hypothesis is that compressing the load can break powder granules into smaller pieces, and this will affect the burn rate of some of the charge.
Has anyone here actually experienced this or is it pretty much a non-issue.
Thanks

[kmon]

Fess, some of my most accurate loads are filled to 103 to 105% capacity. I look at water capacity as a means of comparing cases from different manufactures more than anything else since it is a good standard means of consistent fill of the cases. Some cases between different manufactures I have had are 6 grains if difference in water capacity and a max load for the higher capacity case can be over pressure in the cases with less capacity.

[dega37]

Thanks so much for the replies. Clear and concise. How likely is it to fill right at 80% capacity and have either an over pressure or under pressure problem?

[Klem]

Thanks so much for the replies. Clear and concise. How likely is it to fill right at 80% capacity and have either an over pressure or under pressure problem?

Depends on your choice of powder and how much you put in. Manufacturer tables are a credible source of info here. Quickload, the internal ballistics software will also tell you this information. Or, if you want to ask the forum we can help you avoid any grief.

[dega37]

Quickload seems pretty awesome. The seating depth seems tricky in the app. It appears there is even a checkbox for whether the bullet is boat tail so it seems to not actually know the volume it will occupy. Do you know if this is depth below the neck? Also is load capacity to bottom of the neck or the top of the mouth?

[Klem]

In QL you enter the case capacity by measuring the weight of water to the top, the case mouth.

QL has the boat tail angles for all bullets in its library. It's only new bullets like the Sierra Tipped Match Kings that I have had to enter a new bullet and the diameters of the shank, base and length between (for the program to work out the angle). Once entered that appears on the list with the others.

As you push the bullet into the case measured by overall length QL will calculate the available space in the case for powder, including the angle of the bullet. This of course increases pressure which is also calculated. I don't know for sure but why else does it need to know the angle of the boat tail on new bullets?

[Fess]

Thanks so much for the replies. Clear and concise. How likely is it to fill right at 80% capacity and have either an over pressure or under pressure problem?

Klem's rule of thumb is a good one. Secondary Explosion Effect can be catastrophic. Nevertheless, there are a few exceptions. Hodgdon's site has a section for Reduced Recoil loads and Cowboy Action loads that give some info for SPECIFIC powders being loaded down to 60% density. In general, follow load guidelines published by reputable sources. It is absolutely possible to have an overpressure load at 80% if you use the wrong powder. That is how some kabooms happen. Every now and then someone accidentally uses the wrong powder.

[dega37]

Awesome all, thanks for the replies. Wrong powder at 80% makes sense.

[mtnlvr]

The 80% "rule" must only apply to certain recipes. Hell, my go to 45acp load is well under 50% fill, is plenty accurate, and proven safe for the 10's of thousands of rounds of it that I've fired. Maybe it's only a bottleneck cartridge thing? Does anyone really have any definitive, proven resource for this phenomenon?

[Sinclair]

Years ago in the American Rifleman there was a warning that the majority of firearms damaged or destroyed by reloads were by squib loads. An over pressure load has warning signs and it usually takes more that one firing to go kaboom in your hand were as the squib load happens only once.

mtnlvr: My father loaded 9mm and .45 loads so slow that you could see the bullet in flight. Some people are just lucky. I hope that I never have to stand beside you on the pistol range!

[mtnlvr]

Ahh... But it's not the squib that gets you... It's the next round! I've had at least three squibs in my range fodder reloads (only in the 45acp). Never did they cause any damage, just some down time to strip the barrel out and push out the blockage. If you're paying attention and don't rack another round in, it's not that big of deal (providing its just while shooting for fun).

[Klem]

The 80% "rule" must only apply to certain recipes. Hell, my go to 45acp load is well under 50% fill, is plenty accurate, and proven safe for the 10's of thousands of rounds of it that I've fired. Maybe it's only a bottleneck cartridge thing? Does anyone really have any definitive, proven resource for this phenomenon?

mt,

A squib load is one where the projectile doesn't have enough force to exit the barrel. This implies low pressure. The danger comes when the next one is fired behind it. What I am referring to with SEE and Flash-over is the opposite: over-pressure.

No doubt handgun rounds are a different situation to rifle cartridges given we load slower powders into rifles and there is more surface area exposed exposed to primer spit when it is horizontal