Weighing Brass vs. Case Capacity

When M. L. McPherson worked on the QuickLOAD's English language manual, he indicated that the built-in capacity estimator was good to about 1%. It depends both on the case weight and a number of external dimensions being measured and entered.

The reason the dimensions are needed is that if you have a cartridge producing more than roughly 30,000 psi peak pressure, the peak pressure depends on the volume the case fireforms to in the chamber, and not on its sized volume. Since chambers aren't all the same size, and this means length and diameter both, it follows that even the same case fired in two different guns can has different capacities for pressure purposes.

For case fill purposes, sized volume is good enough. But, guess what; cases coming out of different chambers don't all size down the same amount in the same sizing die. The ones that start bigger spring back out a little more coming out of the sizing die. The Sierra X-ring newsletter once pointed out that cases from some guns can't be resized enough to fit a tight chamber even with a small base die. If you use once-fired brass, you'll likely have run into it.

IMHO, you're better off just doing the water weighing thing with an average case from any given lot and getting a real number to use that way.

Two points. First, my mind wandered a bit on the post and I got to just addressing wanting to know the absolute case capacity. If you aren't using QuickLOAD, that's less directly helpful, but not zero help. Look at this thread from a couple of days ago about case capacity to get the numbers on the significance of weight difference. Bottom line, with 70:30 cartridge brass having a specific gravity of 8.53, it works out for most rifle loads that it takes about 1.4 grains difference in brass weight to have the same effect on pressure as changing the powder charge weight 0.1 grains.

From the above, if you have a load that is insensitive to charge weight changes over a span of 1.0 grains, and you throw charges over a span of half a grain, then it will take a brass weight difference that exceeds 7 grains to drag the most extreme charges you throw into opening your groups up. Loads that insensitive are not uncommon, but I've had a couple over the years that were much more forgiving (2.5 grain span for one .308 load of Brigadier 3032 uisng the 155 grain Sierra Palma bullet) and less (.4 grains for another load of AA2520 with the 168 grian SMK). This echoes some of Hatcher's results with coarse and fine grained powder from developing National Match loads, the latter being touchier. I considered the latter unhelpful as it didn't have good immunity to temperature change and was picky about brass capacity uniformity, too, of course. The former was immune to all conditions I could throw at it. It was great, so they up and burned the Brigadier powder plant in Scotland down to be sure I couldn't get any more.

Dan Newberry's OCW site is devoted to identifying forgiving loads, if you want to read about doing that?

If you are just sorting brass within a given lot, and it all has the same load history, weight comparison likely tells you all you need to know about capacity for all but bench rest match purposes, and you probably only have to sort it once in the life of the brass. Just be aware cases grow capacity over time due to trimming, which weight change catches, and head brass migrates rearward, which weight change does not catch, and which is not reliably uniform. Denton Bramwell's article on why case head expansion and pressure ring expansion are not a reliable pressure measures shows this indirectly. He measured head expansion happening sometimes at very low pressures, like around 32 kpsi for some cases, and not until 65 kpsi for other cases all from the same lot and with the same load history. Likewise, he shows pressure ring expansion erratic. What this means, and slicing enough cases will show this to you, is some cases have more of their brass in their heads than others or else have it more on one side than others, allowing, as Rocky said, more expansion under pressure due to the resulting thin places.

I don't recall reading of anyone doing repeat loading of cases to see whether muzzle velocity variation for a load tended to follow particular cases or not? That is, whether you could identify cases within a lot that tended to produce the same MV based on how they expand? It would clearly be of no consequence at shorter ranges to most kinds of shooting—say to 300 yards and under—but the 1000 yard and bench rest shooters would surely like to know?

I recall Merrill Martin reported finding one case in an otherwise matched set that always shot slightly lower than the others (this was bench rest shooting). He measured carefully and found it to be the only dimensionally perfectly uniform case in the batch. The others were all thinner on one side than the other and presumably expanded a little more due to the thin portion, thus lowering barrel time slightly. That apparently let the muzzle move up a little higher by the point of departure. I don't recall him reporting velocities.

So, fussy is as fussy does, but some of these tiny changes actually do have measurable effect, at least in some loads in some guns. If you are in a situation where it is enough to matter, then you want to know about it. As always, the final bottom line is testing in your own gun under your own shooting conditions to see whether any effect is apparent or not?

I am not a fan of blowing off precision with the declaration that "the gun out shoots me as it is". I've seen too many beginners in service rifle match shooting say the same, only to find their scores jump up ten points when someone lends them a match rifle. You only have to turn ten almost-makes-its into scratch hits of the next higher scoring ring for that to occur. So, I'd rather err on the side of precision. I can prove statistically that it takes far less time and fewer rounds to first overdo exactness, then eliminate each required step until you isolate score affecting factors, than by gradual incorporation of such steps. But, as always, the key is your purposes. A deer will most often be less sensitive to that short difference between an almost-makes and a scratch hit than a scorekeeper is.

Depends what range he fired the groups at and with what bullet? Change in case volume affects peak pressure and MV. An MV variance that produces a tenth of an inch drop on a one hundred yard target can produce half an inch on a 200 yard target and two feet on a 1000 yard target, depending on the bullet BC and starting velocity. So, again, it depends on your purposes?

If a load has reasonable charge immunity and the range isn't too long and the charges are precise and good match bullets are used, it may be that case volume change with age won't present a problem during the life of the case. And keep in mind that these groups are five shots in a row. If you made up a group of five shots consisting of one shot from a case that is new, and four more each taken after another 10 rounds had been put through the case, you may well see a POI change very substantially at long range. The point of weighing and tracking load history is to try to keep cases as similar as possible at any given point in time, not from start to retirement.

The best tuned and most carefully assembled load in the world won't make a poorly assembled gun into a shooter. But neither will it make it worse. Working up a handful of match cases, or even just using gauges to select one good case, then doing your best to develop a tight shooting load is cheaper and easier than customizing a gun. So it makes the most reasonable starting point. Once you have it, if the gun doesn't meet your requirements, start with bedding, recrowning, bolt lug lapping, firelapping, better trigger, better sights, or other steps that occur to you could help get it shooting. Once it meets your expectations, then start slopping the loading specs. Try sloppier powder measuring, standard vs. BR primers, hunting verses match bullets. Try brass as issued verses brass that's prepped, or FL dies verses your Lee Collet Die. Try to find a load of cheaper or easier to meter powder. Try moving the seating depth around.

Find what matters to your shooting needs and do only that.

Well Nick, you're certainly full of useful suggestions. And of course, I suppose that for an experienced reloader, alot of this is just part of the process.

I wonder though (not intentionally trying to get too far off my own topic), about an ideal procedure for working up a really great load in any given rifle. Again, while alot of these things may make an extremely little difference, everything compounded can sometimes make a huge difference.

Like souping up a truck for instance. You can upgrade exhaust and injectors and air intake, and turbo, fuel lines, fuel pump, etc. All these things may make some power difference, though maybe not enough to seem worthwhile. Then all of a sudden you introduce a tuner (to optimize the amount of fuel delivered and the time it is delivered, and the paramaters of the ECM), and it seems to make a HUGE difference, because you have all of the support systems already in place for it.

So, if I'm working up a load, and I really want to play with things to see how good I can make it. I'm going to start off first of all always using a lower end charge, and experiment with bullet types/weights to see what agrees the most with my specific gun (in this case I will put the emphasis on accuracy, though of course it could be whatever you deem important). Then I'll use that bullet as a constant, and start working up a load for it, to see what it likes best there. Then I'll try different types of powder, working a load up on them to see which performs best there. Next I'll back my loads down to experiment with various makes of primers, and work a load back up on all them to see the difference.

Keep in mind of course that this is all hypothetical, and I don't necessarily plan on doing all of this...just curious about the process. Is this the type of order one would attempt to work it out in, or is there a more efficient way to do it?

My groups are usually shot at 100 yards unless other applications apply. I use hunting bullets because that is what I am going to use most of my rifles for. Mostly Nosler and Berger VLD's. Most of my rifles shoot 1/2 MOA or better too.

We were just talking about this yesterday with my gunsmith, Corky (also my fiancee's grandfather). He shot back in the 60's against guys like Ed Stolle, Ferris Pindell, and was really good friend with Robert Hart among many other big names. He was telling us the story about how the first time he met Ed Stolle he pissed him off cause Corky was relatively new shooter on the field and was out shooting most of the top guys. They asked him about his reloading practices and weighing cases. He said he weighed and sorted everything to ensure consistency. Ed said no need for the extra work. Late one night Corky shot 2-5 shot groups @ 1 or 200 (can't remember which), one sorted another not sorted, showed it to Ed and he got all flustered, he proved Ed wrong. Then he invited him over for a beer and they were good friends after that. These guys were shooting 5 shot groups @ 100 in the 0.2"'s to 0.4"'s and 200 yard groups around 0.4" to 0.6" at the time, which is still really great shooting. He has the national match score board up in his shop from that match. He got 6th I believe overall.

Granted, could have been other factors, but how do you know? Unclenick is also right that at these shorter ranges it is not going to make a large difference.

So needless to say whenever Corky talks about shooting, reloading, and technique, I listen!

Also my groups are never fired with a brand new case, always at least once or twice fired brass out of my gun.

Hi Gary.

Good to have you on the forum. Seems like you should have some good insight into how things work, and I hope you stick around for a bit to help enlighten me!

I can honestly say that I've never heard of neck thickness causing vertical spread and the length of the shoulder (or should I say, length TO the shoulder) causing horizontal spread. Then again, I'm just a baby in the world of reloading. Not yet a newborn, in fact.

Just curious if you have any stats to back that up, or know of any tests done? I'm not saying I don't trust your advice, of course...just curious on the actual effect these things will have on group size. On that same note, I'd be curious to shoot factory ammo in my rifle (say maybe 3 groups of three shots each, and at, say 300 yards so any differences are noticeable), measure group sizes, then full length size that same brass and reload it as close to factory as possible, and measure those sizes. Next neck size it, reload as close as possible to factory, and measure those groups, then work up a load and measure those, then weight the cases and measure those groups...and on and on... and just see how much difference each step actually makes. Then someday I'd like to do the same with accurizing my rifles, measure groups after each upgrade to see what difference they make in the real world.

That's why I'm not satisfied with this bit about it "not mattering" how precise you are in reloading...after all, if all of these steps truly didn't make enough difference to be worried about, then why do most folks handload anyway, or even accurize their rifles?

Where to start?

Stats, no. I have reloaded for just over 20 years. I am lucky enough to have been taught how to reload and how to shoot by an army green sniper from the Viet Nam era. One thing that really got me hooked on reloading was being able to *MAKE* dang near any rifle shoot MOA, "no matter how much effort, and head scratching" it took me. These days my enthusiasm has shifted towards getting good trigger time in on my one and only one favorite rifle

-- Neck Tension: if your necks are not equally tensioning the bullet, you will get erratic pressures. Thus, you will get erratic velocity. Erratic velocity gives you vertical stringing as you step out in yardage. I would suggest 400 yards as a good test of "Stringing" both vert and horiz. 300 would be min.
-- The shoulder bump thing is harder to explain. But if you take 5 cases and reload them, say 15 times (308 win) at some point you will notice that with just neck sizing only they get "Tight" to chamber. When this happens you will begin to notice groups doing odd things. Anneal the necks at this point, then bump the shoulders back.

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Good shooting,
Gary