Effects of Case Weight Variation on Accuracy

[South Pender]

I'm starting to load for a .222 Rem. and am using Lapua cases. I weighed about 100 of the cases and found them to vary between about 92.0 and 94.0 grains. At this point, I'm wondering whether this much variation in 5 cases shot for group would cause variations in internal capacity enough to have any, or much, effect on accuracy. In other words, suppose I had 5 rounds shot for group in which the case weight varied between, say, 92.5 gr. and 94.0 gr. Would this much case-weight variation alter the internal capacity enough to produce sufficient velocity variation to affect group size?

If the answer is Yes, then I could sort the cases by weight and only include cases with very close weights in each group of 5 to be shot for group. For example, one set of 5 rounds might be using cases that range between 92.0 and 92.5 gr., whereas the next set of 5 rounds might be using cases varying between 92.5 and 92.9 gr., etc. But this is adding quite a bit of extra time to my reloading. Do you guys think that this would ensure smaller groups over forgoing this sorting and just using the cases as they come?

 

[Texas10]

If you think it would help to cull or sort by weight group, I say do it. Confidence in one's performance is, in my opinion, a bigger factor in getting the results you want than most of the little tuning tricks one chooses to employ.

Personally I have sorted by weight groups but I have not noticed any difference, probably because in my case, the nut behind the bolt is still the biggest factor.

 

[South Pender]

One thing that I could do, I guess, is to sort them just once into, say, (a) lowest-weight quarter of the cases, (b) next heaviest quarter, (c) next heaviest quarter, and (d) heaviest quarter of the cases. That way, each quarter would have 25 cases that varied in weight by probably no more than about .5 gr. If I could figure out a way to mark them so that I didn't have to sort them over again each time I reloaded them, this might be a fall-back position. The case head on a .222 case is so small that there's not much room to mark anything there, and any mark on the case head would probably be removed when the case was fired. What have you guys come up with to mark sorted cases? Maybe a touch of nail polish in the extractor groove?

By the way, Texas10, I completely agree with your last comment. This nut behind the bolt is far from an accomplished shooter....

 

[243winxb]

Yes. After first doing a full BR case prep.

A chamber has a volume. This volume runs from the bolt face to where the bullet seals the bore. Any time a different weight is put in the volume, pressure changes. I did a simple 1 time test using 100 win new brass , 243 win caliber in my Rem 40x. Brass seperated by 1/10 gr. I took 5 heavest, and 5 lightest. Shot groups with each. Then took 3 heavy & 2 light shot group. Then took 3 light & 2 heavy shot group. You could see the difference. It pays to weigh brass for an accurate rifle IMO.

Make groups of 3/10 th grs.

FL every loading. If mixing neck sized one time a nd Fl the next loading , case volume is different, requiring different powder charges.

 

[Ned Ludd]

South Pender - Load a few rounds each of the very highest and lowest weight cases, and see if you can shoot or detect any difference between them in terms of grouping or velocity. If after doing that you decide it will probably make a difference for you, the simplest way to sort cases by weight is generally going to be something like light/medium/heavy. However, I will typically weigh about 50 cases or so first and record the weights. You can then do a little counting and eyeball approximately where the weight range cutoff points should be to give you fairly equal numbers of cases in each weight group. The distribution will likely not be uniform and having one weight group with too few cases to load up for a match somewhat defeats the purpose. No matter how you sort cases by weight, there will always be some outliers where case weight does not accurately represent case volume. Nonetheless, sorting cases by weight can allow you to achieve more uniform case volume with a relatively small effort than if you didn't sort cases by any method at all.

 

[jepp2]

Do you guys think that this would ensure smaller groups over forgoing this sorting and just using the cases as they come?

Two things to keep in mind:
1. Case weight isn't exactly equal to case capacity, so sorting by fired cases of similar volume will be more accurate (and much more work to accomplish)

2. the following was from "Unclenick" on TFL forum regarding the benefit of sorting 204 Ruger cases based on weight:

"Cartridge brass density is 8.53 gm/cc, according to Matweb. That means 0.853 grains, in identically trimmed and primer pocket uniformed cases, will represent 0.1 grains difference in powder space. Since pressure goes up exponentially with increase in powder charge, this is about like a 0.05 grain charge difference. Not usually a significant error. If you charge cases ±0.1 grains, then figure cases ±1.7 grains are roughly equivalent to that."

Since I dispense powder to 0.1 gr accuracy, if I sorted my ~100 gr. cases by 2 grains weight, I'm getting the maximum benefit. So when I have a situation like your 100 cases of Lapua, I just break them into 2 "batches", the heavier case weights and the lighter case weights.

 

[TAJ45]

Just a suggestion for easier implementation.
If you have a group that is short on count needed for a match, use those for practice.
Save the "good ones" for the main job.

 

[Bindi2]

People go to a lot of trouble with case weight but do absolutely nothing with powder except weigh it. The powder volume mass changes with the same weight negating the previous case work of weight sorting. Volume sorting cases is better but the volume of powder to case volume is more critical.

 

[South Pender]

People go to a lot of trouble with case weight but do absolutely nothing with powder except weigh it. The volume mass changes with the same weight negating the previous case work of weight sorting. Volume sorting cases is better but the volume of powder to case volume is more critical.

It would seem to me that volume sorting cases would be a ton of work; I guess you'd use water. However, wouldn't you expect a close to perfect negative correlation between cases sorted by (X) weight and (Y) volume? I don't understand your statement, "the volume mass changes with the same weight...." Wouldn't two cases of exactly the same weight have identical volume (assuming, of course, that their external dimensions are also identical--via resizing for example)?

 

[Bindi2]

The powder volume mass changes with the same weight. There is a post on here somewhere that showed volume changes with the same case weight. Case weight has little to no connection to case volume. The powder volume to case volume is the critical measurement.

 

[F Class John]

I will say it’s a mixed bag at my club. But one of our top guys did some lapua testing. He sorted brass and the overall consensus was that it had almost no effect on much of anything. Primer and bullet sorting had more effect than sorting brass did by a decent margin. YMMV but that’s what we saw. All that being said I still sort my brass because it gives me a little extra boost of confidence however stupid that sounds to some.

 

[MislMan]

The utility of this effort would be based on your intended discipline for shooting (hunting, f-class, benchrest score, benchrest group) and where to are in the hierachy of that discipline. Beginners (such as myself) probably shouldn’t worry this since it’s doubtful they shoot well enough to see the differences. Those at the highest levels would likely see the differences and make the choice for or againest the added steps based on their needs.

 

[Texas10]

It would seem to me that volume sorting cases would be a ton of work; I guess you'd use water. However, wouldn't you expect a close to perfect negative correlation between cases sorted by (X) weight and (Y) volume? I don't understand your statement, "the volume mass changes with the same weight...." Wouldn't two cases of exactly the same weight have identical volume (assuming, of course, that their external dimensions are also identical--via resizing for example)?

And that's the big assumption regarding brass weight vs case volume. The brass can indeed be redistributed within the confines of the outer case dimensions enough to vary the case volume significantly. It all depends upon where it goes. For instance the extractor grove can cut very differently effecting the case weight, but not the volume, or the case internal dimensions can be much tighter yet the outer case machine work may make up for the change in weight thereby offsetting the material increase internally.

 

[Ned Ludd]

The powder volume mass changes with the same weight. There is a post on here somewhere that showed volume changes with the same case weight. Case weight has little to no connection to case volume. The powder volume to case volume is the critical measurement.

This is simply not correct. People post this sort of thing all the time and it's simply not true. I'm convinced that in most instances, this misleading opinion results largely from user error in accurately determining water volume. The simple fact is that the only places in which brass that has been fully and uniformly expanded by firing at 50K to 60K+ psi in a rifle chamber can change in thickness without affecting internal volume are in the extractor groove and the primer pocket. In fact, it is only the variance in the volume of the extractor groove and/or primer pocket between different cases that really matters. The total variance between the extractor groove and primer pocket volumes of different cases as a percentage of the total internal case volume simply isn't that large. It just isn't. Although you will always find some "outliers" where the data point of case weight (y-axis) and case volume (x-axis) does not lie directly on the scatter plot trend line, determination of correlation coefficients supports the conclusion that case internal volume generally exhibits a good linear correlation with respect to case weight.

Further, powder mass doesn't "change", and I'm guessing "powder volume mass" is a term that you made up for the purpose of this discussion. Assuming that a given mass of powder has not been artificially compressed or unevenly packed in such a way as to give a dramatically different packing density, the major change we need to be concerned with is the volume of the pressure cell in which a given mass powder is ignited. As internal case volume decreases, uniform ignition of a given mass of powder will create higher pressure due to the expansion of the same amount of gas in the smaller internal case volume (i.e. pressure cell). If a specified mass of powder is unevenly packed, resulting in non-uniform density from case to case, minor differences in internal case volume are the least of your worries; the pressure will not be uniform even if all the cases have exactly the same internal volume. Although it is possible to alter the packing density for a given mass of powder in a case, for example by using a longer drop tube or a vibrating table to settle or compress the powder, it will generally be relatively uniform from case to case, even between those of slightly different internal volume, as long as the reloader's technique is fairly consistent and the load is not highly compressed once a bullet is seated.

The whole point of this thread is the use of a much simpler and more expedient method of weighing cases as a surrogate for determination of their actual water volume. Is it perfect? Of course not. As I mentioned earlier, you will always find a certain number of outliers in a scatter plot of case weight versus case volume. I have never observed ALL the plotted values to lie directly on the trend line. However, most are not "gross" outliers, and plot reasonably close to the trend line. Close enough, in fact, that that the correlation coefficients indicate a strong linear correlation between case weight (x) and case volume (y) as I mentioned above. As a result, sorting cases by weight will typically result in more uniform internal volume than doing nothing at all. Whether the time and effort of doing so is actually justified by the relatively small benefit probably depends on a number of factors, including the brass brand/manufacturer, inherent accuracy/precision of the rifle/load setup, and the relative skill level of the shooter, themselves. In other words, everyone need to do their own testing and decide for themselves whether it is worth the effort.

 

[Bindi2]

Sorry to be a fly in your ointment the mass or volume of a powder does change or can be different for the same weight. And I will say again case weight is no indicator of case volume . This may not be true for all case manufacturers. I binned Lapua before I got that far in prep and use.

 

[Laurie]

If a specified mass of powder is unevenly packed, resulting in non-uniform density from case to case, minor differences in internal case volume are the least of your worries; the pressure will not be uniform even if all the cases have exactly the same internal volume. Although it is possible to alter the packing density for a given mass of powder in a case, for example by using a longer drop tube or a vibrating table to settle or compress the powder, it will generally be relatively uniform from case to case, even between those of slightly different internal volume, as long as the reloader's technique is fairly consistent and the load is not highly compressed once a bullet is seated.

This is an interesting subject in itself and and actual results may be counter-intuitive. Way back in the 50s the H P White laboratory did some work on this using 30-06 and a compressed load of IMR powder, most likely 4350. It found that a careful but straightforward pour into the case that saw the resulting fill compress the charge once the bullet was seated consistently gave smaller ES/SDs than more settled and un / less compressed charges from the use of a super-long drop tube and even less still than settling the charge using vibration. The conclusion was that over-settled kernels impeded primer flame penetration of the charge and made ignition less rather than more consistent. When I started handloading many years back many manuals warned against compressed loads with ball type powders. The usual argument (if any reason was given at all) was that this type relies almost entirely on its surface coatings to control the initial burn, much more so than extruded types which used kernel shape and size and internal hole diameter more, and if you smash up the little balls you get very different burning speed characteristics. This issue / argument / advice has disappeared from nearly all recent manuals except for Norma's. Norma still advises against compressed ball powder loads but on the basis of reduced ignition consistency and efficiency as compression sees already closely aligned kernels packed too closely together and lacking spaces between them impeding the primer flame. This may or may not happen in real life, but it resonates with what the H P White people found with IMR 'log type' powders and vibratory settling.

On the basis of the original question I'm much less bothered about case weight variations than I might have once been but still do some weight batching almost as a talisman - it's an easy once-off exercise, so why not? However, should I find very large spreads I'm not at all reassured about the manufacturing quality even if case neck thickness is uniform. I've always wanted to do a case comparison exercise and look at whether or how much case-prep techniques give any noticeable benefits and have finally got the bits together to do this over the next two or three years primarily with two rifles and cartridges - 243 Win in a factory (Howa 1500) varmint-barrel sporter and 260 Rem in a reasonably good precision rifle (FN SPR / A5 rebarreled with a Bartlein Heavy Palma profile 28-inch tube and throated long for 3-inch COALs with 130-140gn match bullets.

There is a better choice of brass available these days than there has ever been and standards have generally improved substantially. Even so, with the 243 I went out and bought 100 Serbian PPU cases, the cheapest make on the UK market and very popular with our deerstalkers and many recreational range shooters on simple price grounds. (Its factory ammo works well enough on deer too I'm told.) These 100 examples produced the largest weight spread of any case I've looked at in a long time, 5.7gn between extremes on a median weight of 174.2gn or a 3.3% variation. (In the days when all advice on precision shooting recommended case batching by weight, 1% variation on the average was advised as the maximum desirable.) So I managed to batch 50 cases with weights in the middle of the spread covering 2gn or so and put the others with the low and high outliers together in a second 50-ct ammo box with the near 6gn spread. Once I've got a load the rifle likes, most likely with the old 70gn Sierra MK, it'll be interesting to see if any difference can be seen either on paper or the Labradar between the 'good' and 'bad' lots. In this kind of rifle I'm pretty sure a field shooter will never see a difference at deer or fox shooting type distances ............. but who knows?

One other and final comment on this weight issue is it reminded me of a tale told to me by John Carmichael former managing director (now retired to Canada) of HPS-Target Rifle Ltd. and the driving force behind one of only two custom ammo loaders that I know of in the UK, JHC Target-Master ammo. (The other is Kynamco which specialises in loading hunting ammunition in obsolete cartridges, particularly those big bore African numbers for both bolt and double rifles.) John was a top flight UK international Target / Palma rifle sling shooter for decades, 'Match Rifle' too, and then switched to F-Class when it arrived. UK 'Target Rifle' was characterised for (too many) years by the GB NRA's insistence on the use of centrally procured and match organiser issued military specification 7.62mm ball some of whose lots left a lot to be desired in consistency, especially in 800-1,000 yard match stages. All sorts of 'dodges' and 'wheezes' such as super-slow rifling twists were used to try to overcome ammunition deficiencies. Some worked and some didn't. John told me that after small battery powered electronic scales appeared, he and a few people starting weighing and batching cartridges after they were issued at the beginning of the larger fixtures where over 100 rounds might be involved. The most consistent ones were kept for long-range match stages, the outliers for short-range and/or 'sighters'. He assured me this did work. Whether this results from reducing case weight variations or the sum of all component weight variations I don't know.

 

[Mikemontminy]

Here’s another test that could be done to see if case weight matters. Pick several different weight cases and load each one at the range . Mark or keep track of each case and fire case 1 at Target 1 and same for all the others until a 5 or 10 shot group forms on each target with that particular case. Also keep track of velocities with each case that way the group size and volicities can be check for each case individually. With everything else being equal you could see if case internal volume or weight makes a big enough difference to matter.

 

[Liven The Dream]

I weight sort my case for all my Rifles ,Palma ,F/Class and Hunting . Makes me fell positive .
I use a 3 weight system LT,AV&HV I put then in 50 or 100 round Boxes. I also track them as Times Fired.
When they are new they get a complete Prep job.
As state makes me fell good about my Ammo....

Now for another good friend and Palma Shooter His system.
All New Lapua Brass SP , Prime add Powder add Bullet shoot.
Note He is a High Master and has been ask to try for the Palma Team.
He hold some records at local ranges ? Go figure .

He will open a New Lot of Varget and use His go to Number 46.5 grs.
If the load shows signs of being to Hot ! he will load the next time with less powder ?

What does all this point to ?? Your Dammed if you do . Your dammed if you don't.
Or It's the Indian not the Arrow ??

 

[243winxb]

I've always wanted to do a case comparison exercise and look at whether or how much case-prep techniques give any noticeable benefits

I have, sort of , in 243win with a 40x and Rem 600 Mohawk carbine. But not a true brass only comparison. Using a fl bushing die helps also.

Comparing unsorted brass with standard dies to benchrest preped brass & bushing die got rid of the group killing flyers in both rifles. Win brass in 40x and Rem brass in the 600.

The powder volume mass changes with the same (case)weight

A Military 5.56 test showed a 4% increase in case volume between virgin rounds and when firing the ammo.

The chamber volume is increased as a result of a combination of the brass cartridge case expanding and the projectile moving forward to engage the rifling . Part of this difference is due to the fact that the brass cartridge case expands (4% volume increase) during the ballistic event.

I dont see the internal case volume changing when fully fire formed to the chamber. But add in spring back and the fact that brass is always flowing/changing, only the target knows what works.

https://apps.dtic.mil/docs/citations/ADA456635

 

[Laurie]

Now for another good friend and Palma Shooter His system.
All New Lapua Brass SP , Prime add Powder add Bullet shoot.
Note He is a High Master and has been ask to try for the Palma Team.
He hold some records at local ranges ? Go figure .

That's a pretty common practice these days with Lapua and other better makes. I still like to check weight and run a Sinclair flash-hole uniformer through new Lapua, Norma or Peterson brass though. Very, very occasionally, a rogue case or two appears. In one [pre 'blue-box'] box of Lapua 223 Rem brass I was preparing some years back I found three examples where the flash-holes were seriously undersize to the point where it was difficult to get the tool to start and needed many turns before resistance slackened. In this little case that's likely to be disastrous. (The holes were so undersize I wondered if they had been formed with the 1.5mm tool for BR cases and suchlike.)

In much US made brass, although quality and consistency are generally pretty good these days (although I've found like others that Winchester seems to have gone downhill especially with inconsistent and concave primer pockets and oval / oversize flash-holes), punched through burrs on flash-hole exits are usually an issue so a de-burr is an essential chore. I had some early Hornady 6.5 Creedmoor brass from the days when this was the sole make and as well as what were apparently two lots with a large average weight discrepancy between them (which translated to a 0.3gn water capacity difference in fireformed examples), they all suffered from punch spikes to a greater or lesser extent. Two cases had punch ejecta still attached in the form of a full figure '8' in one case and a '6' or '9' in the other which partially or even largely blocking the flash-hole.