Case volume difference

[cdoc42]

I have a Winchester Model 70 in .270 converted to .25-06 with a 26-inch Hart barrel. Lyman #50 lists the most accurate powder in their 26-in test barrel with a Winchester case as H-1000 with 58.0 gr maximum delivering 3038 fps with a Sierra flat base 117gr spitzer.

I have both Winchester and R-P cases, but I'm shooting a Sierra 117gr SPBT rather than the flat base. I wanted to see if there was any difference between the two cases.

I loaded 5 rounds each with H-1000 but stayed 2.0gr under max at 56.0gr, CCI 200 primers, and seated the bullets 0.02" from the lead with an OAL case-base to ogive of 2.735".

I tested the speed with a Magnetospeed chronograph and reset the position after each shot fired as it jumped about an inch with each recoil, and I alternated the shots with each of the cases.

To my surprise, the 5 R-P cases delivered an average of 3118 fps (ES 44) while the Win cases almost matched the Lyman numbers with an average of 3049 fps (ES 54) (recall this is 2.0gr under the Lyman charge).

I weighed two fired cases of both manufacturers, then filled each of the 4 with water until I produced a meniscus. Weighed again, the volume in the R-P cases averaged 68gr while the Win cases averaged 69 grains (rounded off).

Is it possible that the larger Win case volume produced less pressure to account for the lower velocity?

I suspect the difference in Lyman's 3038 fps in Win cases at 58.0gr vs. my 3049 fps with 56.0gr is more complex, but what might be the effect of my using a boat-tail vs Lyman's use of a flat base?

[44 AMP]

there is no way to take into account how much of a factor it is, but one must take into account the fact that your gun and the test gun are different guns. No matter how close to the same you make your gun and ammo to what was tested, it is going to be different, and that difference can account for differences in velocity sometimes surprisingly large ones.

I have seen three different guns, all with the same (nominal) barrel length, firing the same ammunition (same cases, primers, powder charge and bullet) and still have a velocity spread of 100fps between the fastest and the slowest.

We expect loads with less powder or fired through shorter barrels to be slower, and while this is usually the case, it is not always the case, and when "the stars line up just right" the opposite has been known to happen.

Modern chronographs are wonderful useful things, but they also are frustrating "tattletails" showing us things that actually do exist when we had no idea and just assumed what was going on. Sometimes, ignorance is actually bliss and enlightenment is disappointment.

Friend of mine absolutely loved his .270 Wby. Until he got a chronograph, then he loved it a lot less when he found out that his rifle was slower than the books said, and his 22" .270 Win actually beat it by about 30fps.

While this is an insignificant amount, my point is, that different guns CAN (but not always will) do different things and sometimes not the things you expect.

Don't get so wrapped up in the trees you fail to see the forest.

[tangolima]

Difficult to compare with the test gun. With the case volume being the only variable, smaller volume leads to higher pressure and higher MV.

68gr to 69gr H2O is 1.5% up. 3118fps to 3048fps is 2.2% down. I expect less variation in MV. But it is not too much out of whack. It could be due to too small a sample size. Don't know how many shots OP test fired.

-TL

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[jmr40]

Is it possible that the larger Win case volume produced less pressure to account for the lower velocity?

That is exactly what happened and why I don't mix brass when reloading. I load for 308, 30-06 and 6.5CM. IME Hornady, Rem, Winchester and Nosler brass are close enough that I can use the same load data.

There will be slight differences in MV and slight differences in POI on target, but velocity is within the specs in my load manuals and accuracy is about the same with all of them. But POI is enough different that group sizes increase if I mix the brass.

Federal and 308 Military brass however has a little less case volume and requires me to work up a different load with a little lower powder charge. Nothing really wrong with Federal or Military brass, but I no longer use it simply because it requires me to use a different load. It's just easier for me.

[44 AMP]

With the case volume being the only variable, smaller volume leads to higher pressure and higher MV.

The only way to make case volume the only variable is to use only one rifle, one load with identical components other than the case, and fire from a machine rest indoors in a temp controlled environment.

The OP stated that he used a different rifle, different powder charge, different bullet and different cases than what was used in the data he looked at.

I suspect the difference in Lyman's 3038 fps in Win cases at 58.0gr vs. my 3049 fps with 56.0gr is more complex, but what might be the effect of my using a boat-tail vs Lyman's use of a flat base?

You might consider that the difference between 3038fps and 3049fps is 11fps and that is only a quarter of the ES 44fps you gave for your load earlier. IN other words, that tiny difference is inside the margin of error of your data. "lost in the noise" and not anything significant.

The effect of using a boat tail vs. a flat base bullet on MV? Its something that is probably measurable, might be significant, might not be.

You might consider that, other than the same weight, the BT and the Flat base are different bullets, and could easily be different enough to produce not just a measurable difference but one large enough to make a practical difference. Could be the bullets bearing surfaces are different in length. Could be they are different jacket thickness, or slightly different alloy. could be other differences not visible to the naked eye, that will be a factor.

seated the bullets 0.02" from the lead with an OAL case-base to ogive of 2.735".

While descriptive of what you did, these numbers are not of any use to anyone else, which makes them essentially irrelevant.

[tangolima]

I thought he loaded the same cartridge with 2 different brass, shot them with the same rifle, and got different MVs.

-TL

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[cdoc42]

With all due respect, 44AMP, you read too quickly. I greatly appreciate your advice and experience but what you posted had nothing to do with my questions. jmr40 and tangolima were on the same page.

My first paragraph clearly noted I was using one rifle. The second paragraph clearly stated my interest was in any difference using two different cases (with the same load in one rifle). I noted that I alternated 10 shots in those 2 cases.

My comparison to the Lyman data was simply for interest because I have found it rare to have my loads matching those in the manuals, for reasons you clearly espouse. The only comparison that I made to the Lyman data was both rifles have 26-inch barrels just in case the velocity difference might be attempted to be explained by different barrel lengths. They used Winchester cases which is where my data coincidentally coincided even though they used 2.0gr more powder than I did to achieve relatively the same velocity, which is another factor seen in rifle differences (as you point out).

That you considered the velocity difference insignificant failed to address the question. I was thinking a difference in pressures generated in two cases of dissimilar volume might explain what I saw in MY rifle. But I questioned if using the same Winchester case, same primer, but a difference of 2.0 gr of powder in Lyman's data to get the same velocity could be explained by my use of a boat-tail vs. their use of a flat base.

I had a previous experience where my son's Winchester .270 had a significantly better group using a Speer 130 flat base compared to a Speer 130 BT. Unfortunately I never graphed them, or I may have been able to answer my own question.

[44 AMP]

I was thinking a difference in pressures generated in two cases of dissimilar volume might explain what I saw in MY rifle. But I questioned if using the same Winchester case, same primer, but a difference of 2.0 gr of powder in Lyman's data to get the same velocity could be explained by my use of a boat-tail vs. their use of a flat base.

My apologies for misunderstanding what you were looking for.

It is possible that the difference you are seeing in your rifle is due to the different bullets.

It is also possible that the difference in case capacity could play a role, as well.

To (hopefully) find out which of those is the major contributing factor, shoot some of the flat base and boattails out of your rifle using the same load, other than the bullets, and see what kind of velocity difference, if any, you get.

My point here is that tis possible to get different results, even in the same barrel, with different bullets, even when the bullets appear identical, and of course flat base vs. boattail obviously aren't identical.

[Unclenick]

cdoc42,

Standard SAAMI test barrel length is 24" for 35-06, so you have to allow for the velocity added by 2 more inches of the barrel in most comparisons. GRT shows this barrel length difference, producing a 2% change in velocity with your load. GRT shows a one-grain difference in case capacity, producing a 1% difference in velocity. Your experiment produced a 2.2% from the two cases, but I am going to suggest it is possible that 1% was due to the actual capacity difference and the other 1.2% might be due to start pressure differences due to the Winchester (70:30 brass) and the Remington (80:20 brass) being different alloys with different grip levels on the bullet, plus there may be a difference in neck wall thickness. However, that thought is dependent on your having trimmed the two as-fire cases to exactly the same length.

You didn't mention whether the cases had been resized when you measured their capacity or not. At rifle pressures, it is the expanded volume that determines the case's effect on pressure, so you want to weigh and fill them as-fired. Additionally, you need to confirm they are trimmed to the same length after firing. Then, during filling with water, it is a good idea to tap the case a few times to ensure no air bubbles get trapped in the walls. In the end, if you want a number that is good for the interior ballistics programs, you want no meniscus, either plus or minus. I usually fill until I have a positive one, then dip the end of a bit of kite string in and out until the meniscus is flattened. However, just to find the difference between the two cases, the meniscus is fine if it is identical.

[cdoc42]

Thanks, 44AMP, that seems to agree with the theory that Lyman's use of flat base vs. my use of BT, provides a possible explanation. Now all I have to do is wait for Sierra to increase production to meet handloader demand before we all change to some other manufacturer.

Unclenick, your assessment is, as usual, commendable. I DID use fired cases but I did NOT check OAL after firing, but they were in the acceptable range when reloaded (although not particularly equal). The meniscus, believe it not, troubled me. Should I have one? Should I not? Well, my Pharmaceutical background answered that question, so, yes I did.

Your review with regard to the case composition and neck differences was welcomed and improved my understanding of the variable possibilities. I suspect that I should, therefore, use both new cases, trimmed to equal size, measure capacity, fire them with the desired load, and then remeasure capacity. Should I also measure any difference in OAL due to firing since that might relate to the difference in case composition reaction to fire-forming in my particular rifle? If I simply measure water capacity without that consideration, am I missing a variable? Or, if there is a difference after firing, should I retrim then measure capacity?

All this, to any independent observer, may appear as mundane and inconsequential, but I want to suggest to those who have been in this hobby for only a few years, to recognize that if you do continue in your handloading endeavor, there will be a day when you are having the same conversation. Pay attention to the Chief Indians who sit at the tent and just give advice to the young and enthusiastic braves.

[Unclenick]

Yes, I would check for change in length after firing, though it is usually small, as most case growth occurs during resizing.

I forgot to mention that Bryan Litz pointed out that flat bases are easier to make axisymmetrical than boat-tails are, so they can have better accuracy because of that, though when you shoot at ranges for which wind matters, the lower BC can cost you that advantage pretty quickly. Also, there is considerable force on the bullet base from muzzle blast during the first few calibers of travel beyond the muzzle. Harold Vaughn measured it contributing about 3% of the final velocity his .270 Win was getting. As you might imagine, that much muzzle blast playing off a bullet base has to do so evenly to avoid imparting lateral drift. This is why not only the bullet but the muzzle crown need symmetry for accuracy, and the latter factor is why careful recrowning frequently improves rifle accuracy.

[cdoc42]

Unclenick, you are an encyclopedia of valuable knowledge.

Re: any change in the case size post-firing -does the case get longer, thereby reducing the water capacity vis-a-vis the original measurement? If so, would not trimming after firing and before re-measuring, invalidate that variable created, if any?

[44 AMP]

Re: any change in the case size post-firing -does the case get longer, thereby reducing the water capacity vis-a-vis the original measurement? If so, would not trimming after firing and before re-measuring, invalidate that variable created, if any?

Think about it for a moment, you know the answer, already...

The pressure of firing expands the entire case to the limits of the chamber. When the pressure drops the brass springs back slightly, so its not "hammer welded" to the chamber and can be extracted.

The fired case is now larger that the unfired case. It can be longer, it can be fatter, and the shoulder and case head may now be a different distance apart.

Therefore, its logical to assume that since the case is larger on the outside, the inside might have changed size a bit, too, since it was pressure on the inside that expanded the case.

Uncle Nick mentioned that most case growth comes from resizing, and that is true, a couple of ways. You might be stretching the case a bit dragging the expander ball through the resized neck. However the main reason cases get longer is because you are resizing them. The die is literally squeezing a metal cylinder all around its circumference and nearly all its length. Its going to get longer as it gets "thinner".

[Unclenick]

Yes, it's extrusion. Expansion in the chamber makes the brass body fatter and longer than when it last was sized. When it enters the die, the sides of the body are squeezed, making the case even longer from head to shoulder, so when the case shoulder meets the die shoulder, the head is still not pressed as far as the shell holder will let it go into the die. The shoulder, being tapered, acts as a ramp for the brass from the shoulder and body just below it to be funneled into the neck as the rest of the body goes deeper into the die. Some of it turns the corner at the neck, lengthening it. Some has trouble making the turn and contributes to the internal donut ring that forms there (unless there is a mandrel to prevent it, as in the RCBS X-Die).

All that said, provided the growth doesn't leave the overall case too long for the chamber, it doesn't create a case volume change any greater than the volume of the brass moved into the neck. For a 25 cal cartridge with a 0.014" neck wall that grows 0.01", that makes about 0.0146 grains of added water capacity down below the seated bullet, which you seat to the same COL regardless of a little difference in neck length. But when you measure case water overflow capacity, that same 0.01" of growth adds about 0.131 grains of water capacity, which is why I asked if the cases were trimmed. It was just for water capacity measuring purposes.

[zeke]

While am not in possession of a 25-06, have repeatedly noted the difference in velocitys using different cases in 308. Winchester cases consistently produce a slightly lower velocity than most others, and am increasing the charge weight slightly when using win cases. Remington was next, followed by Federal and some military cases. The differences in velocity corresponded to case weights. And yes some will point out that case weight isn't definitive in judging volume, it worked in the testing done.

If memory serves, there was one furrin military case that was lighter or similiar to Win, but would have to check the records.

[Unclenick]

Winchester uses a semi-balloon head that makes more room in the case. When I looked at case weights and capacities in the past, I found weight difference correctly predicts about 80% of the difference in internal volume. In other words, it correlates well but not perfectly. The 20% of the weight difference for which you cannot count on a corresponding volume difference is due to differences in the head form below the powder space. That said, when you are looking at a couple of grains of water capacity difference between two cases, say, from 57 to 59 grains, and you use the rough rule to adjust the powder charge by the square root of the volume change ratio to keep similar pressure, it calls for a difference of 0.8 grains of powder for a 42-grain base charge, and 20% of that is 0.16 grains, the error isn't very big and is getting down into the random velocity variation range.

[44 AMP]

And yes some will point out that case weight isn't definitive in judging volume,..

Case weight, like pressure signs, are absolute and definitive indicators.

Provided you understand what they are telling you, and not what most people think they are telling you.

What they tell us is simple, and reliable. They tell us something is different.

That's all. What is different, and how much it is different is something that must be determined using other methods.

For example, one cannot say, with certainty that X amount of difference in case weight means Y amount of difference in case capacity.

It might, but it isn't a constant thing. You'll certainly see a pattern testing different brass, but you won't see a constant, repeatable "this always = that" thing in numerical terms.

[Unclenick]

...And that reminds me, I should point out that my ±20% result was in the 308 Winchester, and cannot be counted on to apply to other chamberings. Moreover, since the volume that determines pressure is the volume as expanded into your chamber, it need not be accurate in all chambers. One's best bet is to fire a handful of cases, trim the fired cases to the same length to make sure neck height doesn't influence the measurement, and then measure the actual water weight they hold and see how that compares to weight differences, realizing it could be a little different in another's gun.

Also worth mentioning, though the difference it makes is not huge, Winchester and the military use 70:30 brass with a density of 8.53 grams per cubic centimeter, while Remington uses 80:20 brass with a density of 8.67 grams per cubic centimeter, so a weight difference in Remington brass will have about 1.6% lower correlation to volume than Winchester does.

[zeke]

The other adjustment am making for Win 308 brass is using a slightly smaller in dia neck size bushing to get the same degree of neck dia red as the other heavier brass. At least the brass in the neck is actually thinner than Fed, Rem or most other military.

[thallub]

90 percent of my reloads are .30-06, .308 and .223/5.56mm.

All my once fired cases are trimmed to length and full length resized. The reloading die cases have lists of cartridge case weights.

Some time ago i made a boo boo while reloading some .308 cases. The bullet is the 150 grain Hornady Interlock. The load is slightly over maximum when using Winchester cases: It is also very accurate.

i goofed and used Remington cases which average 11.5 grains heavier than the Winchester cases. The velocity was over 100 fps faster than with the Winchester cases. The bolt was hard to lift because case material flowed into the ejector hole of my Remington 700 rifle. Primers were very flattened.

So i tore down some reloads.