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Question for all:

Given a constant charge volume such as 10 grains of Unique, is there a rule of thumb or calculation that allows one to judge differences in anticipated pressure/velocity based on variations of load density resulting from case volume modification due to changes of case length? Case length is the only modification at play.

Assume that the 10 grain charge is 90% load density for the baseline case. Assume two subsequent case modifications change that to 75% (longer)and 110% (shorter) respectively.

Apparently not???

There is the possibility within a certain limited range, but the powder burning rate may not be linear over such large variations. Running a few Quickload simulations could answer that question.

It is interesting that you chose a partial load of Unique for your question. Approximately twenty years ago I needed test bullets both above and below the speed of sound, approximately 1140 fps. Using 125 grain bullets, 38 Special cases, and firing in a 357 Contender, I found that 6.5 grains of Unique hit approximately the right velocity. During extended testing I noticed that I could change the velocity from approximately 1100 fps with the powder forward to approximately 1300 fps with the powder back. That was handy for testing. Using the same ammo I could get a barely subsonic shot or a barely supersonic shot at will from the same gun.

The barrel happened to be fitted with a strain gage so I then measured both velocity and pressure. Five shots from muzzle down back to level gave 1100 fps and 18K psi. Five shots from muzzle up and back to level gave 1300 fps and 28K psi. I repeated this test at several measurement workshop where two participants each got to select five rounds from the ammo can. Charley was a hot-rodder and Jack was cautious. The group never noticed my clumsy gun handling until we had a serious discussion regarding powder position during testing.

As Rich Jamison said, you haven't completed your load development until you have a full case of powder. If you aren't convinced, just measure standard deviation with half the rounds powder forward and half with powder back. The reason for the required "SAAMI Twist" during tests suddenly becomes obvious.

While I have great respect for QuickLoad, Herr Bromel doesn't talk about powder position or case shape. He does make an essential assumption of "uniform and adequate ignition".

We don't explain. Given you load and conditions we could either validate or destroy any rule of thumb with just a little bit of clumsy gun handling during the tests. It's hard enough to simply measure what happens.

Thanks Ken, appreciate that very much.

Curious. I was just running some pistol bullets in the Whelen this week. 13.0 gr W-231 and 14.0 gr Blue Dot with .357 158 gr jacketed flat points in the 1,400-1,500 fps range. (Of course, I was running them over an Oehler 35!) These are in the 20% load density range. With the powder charge laying flat on the bottom of the case, these produced very good accuracy and low ES in velocities. Maybe I'll get around to testing powder forward/back next time around.

Years ago I fired in High Power matches using the 30-06 national match load. This ammo used about 46 grains of IMR4895, whereas the case capacity was at least 60 grains or so.

We were told that one should always single load rounds the same way each time, for instance by pointing the round upwards, slowly leveling it, and closing the bolt slowly.

If you didn't do this you got substantial elevation changes at 600 yards, enough to lose the match.

Sounds as if it kind of validates the use of case fillers such as Puff-Lon, though most I've talked to suggest its all hog-wash.

Phil

Digital Dan:

The short answer to your question is NO.

There are so many parameters missing that it can't be answered.

The long answer is:

Quite a sense of humor! Nice try.

But just to play along in good spirit:

Your parameters 10 grains of Unique at 90% load density dictate a pistol caliber in the 22 Hornet category.

Case length doesn't have any material bearing on guesstimating. It's at best a SWAG!
________________________________________________

BTW SWAG is a Scientific Wild Axx Guess !

Mr. T, I wasn't trying to be humorous. Let me rephrase: If all else is equal except case volume, is there a predictable trend in pressure/velocity? As there is a somewhat useful correlation between changes in pressure/velocity due to alteration of charge weight. If all else is equal charge weight and load density are very closely related.

When you've worked that out get back to me with a joke I haven't heard.

Thinking about this a bit, Dan...can we also assume a straight walled case?

Don't see why not.

If you do testing on powder position for pistols include TiteGroup if it suits your application. Don't have my notes handy and they aren't sufficient to draw a conclusion anyway, but indications were it's much less powder position dependent than the usual suspects for .44 mag reduced loads. Just like Hodgdon says (imagine that).

To the variable case volume question, I don't know. I do know that when working up from absolute minimum charge weight in .44 Mag with the fastest shotgun powder (TiteWad) velocity at one point increases steeply exponentially. Theory is that you've reached the point where the pressure is great enough for the powder to begin burning efficiently. Point is that if you're outside the engineered operating pressure of the powder a change in case volume may, could, dunno, hold surprises.

Digital:

First let me say that MY Quick Load does address the case variables

However, what is necessary is some specific case ID. Including the caliber and name. In the USA the cases are identified by variable and inconsistent names and nomenclature. In Europe it is the ID of caliber in millimeters x the case length in millimeters.

Fast powders are also being being used by some bold and brave re-loaders in rifle cases for some reason.

So in order to provide dependable answers there should be specifics. So to answer your question case lengths do affect ballistics in some cases. The degree of effect is the missing element.

As an example, in a mid range rifle cartridge, a relative small amount of fast burning powder, lets say 10 grains in a 70 grain case, will lay along the bottom of the case. I have read about cases of poor or no ignition. Some people will use fillers.

Although you did specify Unique, consider that gun powders come in single base *( nitro cellulose ), double base *( nitrocellulose and nitroglycerine ), ball, flake, cylindrical shapes and in a whole range of burn rates.

The degree to which low volumes of fast powders change parameters depends on the above variables. And one set of parameters is not necessarily good for some others.

Good news:

"Assume that the 10 grain charge is 90% load density for the baseline case. Assume two subsequent case modifications change that to 75% (longer)and 110% (shorter) respectively."
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I found a candidate!

It's .22 Hornet brass. Case length 1.185 inches. .257 Grain Hornady.

Usable case capacity 10.85 grains. Total case capacity 16.6 grains.

10 Grains of Unique 177,015 psi !

Lengthening the case is 1.85" x 1.75 = 3.32 inches.

Shortening the case 110% would make it somewhere behind the case rim, 1.85 - 2.035 = minus -.185"

If you need more help just ask.

OK, let me simplify the puzzle.

Two cases, identical save for length. .38 Special (1.155")and .357 Magnum (1.290").

180 grain cast semi-wadcutter (Lyman358429) w/gas check, Lyman #2 alloy seated to same index on the forward driving band. In other words, the only variable is case volume.

Max charge of Blue Dot is used in the .38 Special; 7.3 grains at 16,500 psi.

Same charge used in the .357.

-Is there a rule of thumb that will allow reasonable estimation of the pressure in the .357?
-If so, what is it?
-If so, is this applicable to all powders of a given style such as single base/double base?

Not interested in load program calculations actually. The questions above are straightforward.

Comparison of the .38 Special to the .357 is that the .38 case capacity is 23.1 grains while the .357 case is 25.6 grains. But useable case capacity is around 14 grains. So at the load of 7.3 grains the case should be half full. *( The bullet takes up a lot of space ).

Velocities of the two are within 28 fps.

The .38 amount of propellant burned is 93.26%. Ballistic efficiency 39.7%

The .357 amount is 93.31 %. Ballistic efficiency 39.7%

Thus not all powder is burned. Some is blown out.

The difference in the .38 and .357 is so miniscule that there won't be a rule of thumb to depend on ( for these parameters ).

Interestingly changing bullets has as much as, or even more, influence. That's due to the hardness of the bullets, their construction and the amount of "engravement" forcing the bullet into the grooves and lands.

Since the two are so close together it has to be reasonable to expect that single and double base powders will have variable slight influences. Another aspect of influence much more important is the various burn rates which can be rather surprising in the way they act and burn. It's due to their shapes, types and coatings.

10 grains of Unique was way over the top and an impending explosion. Blue Dot is reasonable.

I shoot warm 45 colt loads in a Ruger SRH 454.

I had the same question, so....

I loaded same powder, primer, bullet in both 45 and 454 brass.

Using 3 different powders, N-110, 2400, 4227, it took 2 grains more powder in the 454 brass to equal the velocity of the 45 colt brass..

Small sample, but needed about 10% more powder in the 454 brass............

Molly, thanks. You have any idea what approximate load density was in the respective cases?

Not off the top of my head..... Monday, after work, I'll dump a little powder in a couple of cases and let you know...

Might get around to it later this evening after a nap.........