Ideal cartridge case powder capacities ...

Alan Rushing

I have notes from years ago indicating some of the ideal case capacities for various bullet diameters. Such as:

< .22 cal. =~ 34-35 grs.
.22 cal. =~ 35 grs.
.224 cal. =< 40 grs.
.25 cal. =~ 50-55 grs.
.264 cal. =~ 56-61 grs.
.27 cal. =~ 56 grs. ? ? ?
.30 cal. =< 70 grs.

I realize there are many variables to include:
1) bullet weight;
2) cartridge case shape and contours;
3) different powders, especially today's much slower burning;
4) barrel variables, including the length. (I am not bashful about using 28-30"+ barrels.)

Both JustC and Nononsense shared info recently regarding 6.5mm of 56-61 grs. and two accuracy sweet-spots of 2900-2950fps and 3075-3100.

Is there much of a variation from cartridge to cartridge with other variables being the same?

I would figure that GBs' BR shooters may well have more recent and in depth experience and info regarding all of the calibers. Will you share?

Thanks!

sandwarrior

Alan,

The numbers you give seem a bit high. Possibly old enough that the only criteria they met was "more powder, more speed" Today we also think of things like barrel life and harmonics. Efficiency has become a huge factor in new rounds. With longer barrels and cases clear full of the right speed/pressure powder we get from 35-40 gr. what we used to get from 50-55 gr. It saves on barrels {some}, is easier to tune the harmonics {better steel these days} and of course with additives to the powder we can actually make it perform what we want it to do instead of having to give up and go slower in speed than we wanted.

And, yes I read those posts by JustC and nn and found them to be useful. But not to limit myself in what I'm looking for in other rounds. In the 6.5 Grendel I found 2550 was the most accurate speed I could send out a 123 gr. Lapua Scenar. The max velocity of 2630 fps was not as accurate. I also found that right around 2900 was the most accurate for the 85 gr. Sierra's. I really like that combo. And I would shoot that combo a lot.

I guess what I'm saying is there are natural nodes of accuracy for bullets. But accuracy nodes in powder come more from barrel harmonics. Sometimes the thickness of a given barrel helps dampen movement from all bullets, sometimes it doesn't. Meaning you will have to find what each powder does in each barrel.

Rocky Raab

Two issues being discussed here. I'll comment on case capacity and efficiency. By dint of many hours of crawling through load manuals and crunching numbers, I came up with a formula for maximum efficient case capacity. Before I type it, please note that EFFICIENT does not mean highest performance. It means most bang for the buck. I calculated it based on the reported boost in velocity per added grain of powder. Where that line begins to level off is the point where diminishing returns take over. Got that?

So, maximum EFFICIENT case capacity is approximately bore diameter in inches squared, times 1000. That case capacity in grains of water.

For .22 bore, that's 48.4 grains. Call it 48. The .308 case will hold about 52 grains, and we know that wildcat .22s based on that case are on the edge of being wildly inefficient. It starts taking lots more powder to get even small increases in velocity beyond that case size.

For .24-caliber, it's 57.6 grains of water.

For .25-cal, 62.5 grains of water. (Note that the .25-06 holds 62 grains, boys and girls!)

You can calculate the rest on your own easily enough.

Caliber in inches, squared x 1000

oneoldsap

Looks like you fellows stopped before you got to the really efficient cartridges , those above .30 Cal. The .35 Whelen being the king of efficiency !

Rocky Raab

No, I said you could calculate the rest yourself because it's so simple. But for anybody even more math-challenged than I am (can't be many, LOL!):

.30-cal = 90 grains maximum

.35-cal = 123 grains maximum.

The .35 Whelen gets all its smoosh-power on less than half that much case capacity, so it truly is a VERY efficient round.

BTW, one other very valid way to "compute" efficiency is to use a ratio of bullet energy/grains of powder used. A standard 38 Special load that develops 350 ft-lbs using 3.5 grains of powder would get an "Efficiency Rating" of 100, whereas the mighty 460 Weatherby gets 6000 ft-lbs but takes 125 grains of powder to do it, and garners an ER of only 48. This shows that efficiency and power are not interchangeable terms.

JustC

Alan,...the thing with the "most accurate" or "most efficient" game hinges on BURN RATE of the powder, which is co-dependant on volume.

You can make a superbly accurate load with pistol or small rifle powder. You will not have the speed when using these powders, but it doesn't mean you won't find an accurate load for 100-400yds. The drop will be huge, but they can be made accurate. HOWEVER, if you choose to venture into these areas, you do so at your own risk. When working with fast powders, you can quickly pass the safe zone, and be into the "blow your face off" zone in short order. YES they can be used,...but they are NOT the most efficient. Efficient means a high load density with appropriate pressures.

I run 47.1gr of RL22 under a 140gr pill in the 6.5x55AI and get around 2950fps which is where the sweet spot seems to be with the 140gr 6.5mm pills

Alan Rushing

Just hello!

Believe that by your definition of efficiency, that is what I am most interested in finding. An efficient load for the cartridges in the rifles that I have is what I've always worked on in the past. I would most generally find two decent "sweet spots" and when possible and practical, I'd chose the lower one for general shooting.

What I've worked for in the past: accuracy and consistency, within wise and safe parameters. My concern with powder burn has been to get that done while in the internal ballistic phase when possible. I have no desire to burn powder and get nothing beneficial from it.

Most of my interest for now has to do with selecting an efficient cartridge case design, that will best cover the bases for me, when pushed through the best barrel, designed for that cartridge and the bullets chosen.

Then, for me to build the rig that will maximize that cartridge's potential for my wants, by choosing the best characteristics for that barrel, and the load-up for that cartridge case and bullet.

I've never before planed-out and built a rifle in the cartridge I have selected. Figure the time to chose physical characteristics is now, before the fact. Thanks for the help here everyone ... so far, so good!

Rocky Raab

That is another - and equally good - definition of "efficiency": a cartridge/case/bullet combination that results in the case being full without either compression or airspace, burns to as near as possible but not over the maximum allowed pressure, and produces the maximum velocity attainable in a given barrel.

JustC

have fun assembling a rifle from the ground up, it is addicting and will soon demand lots of time and money. I never minded it one bit and never looked back at the money I have thrown at it over the years.

I like the 40* shoulders that Ackley used, the brass just doesn't seem to stretch at all, and they look kewlll[8D]