• This Forum is for adults 18 years of age or over. By continuing to use this Forum you are confirming that you are 18 or older. No content shall be viewed by any person under 18 in California.

How much weight variation should be in Lapua 308 brass?

T-REX said:
Has anyone tried to determine what case weight variation is worth on the target? I have seen a lot of claims over the years that it is important and other claims that say it is not important but I can not remember anyone that presented meaningful data either way. The new F Class target has touched off a new arms race and is pushing accuracy to a new level so the answer for F Class might be different than a less demanding target.
Case weight has nothing to do with internal capacity . The machine error and the brass quality can change the total weight.. With a Dasher and a small case capacity. .2 tenth of internal case capacity difference I get as much difference as 2'' of vertical at 1000. Larry
 
Guys, just stop and think about it for a minute. Does it seem reasonable that such a small variation in case volume would make a significant difference on the target? Am I the only one who thinks this is not reasonable? If I am the only one that has a problem accepting this then I am probably wrong. I have a great appreciation for data and would like to see convincing data one way or another. What do you think about a test that would use an order of magnitude (ten times) the case volume variation that we normally experience and see if we could see an effect from that. If we did not see a significant effect from that variation we could conclude that the smaller variation was not significant. If we do see an effect we could at least have something to base our estimate on for the smaller variation. Anyone got another idea as to how to test this parameter? Probably most folks will just say why bother trying to do a test since it is fairly easy just to eliminate the variable and not have to worry about it and that is a reasonable position to take. I have an appreciation for good science and enjoy that aspect of the sport as well as the shooting.
 
Have you ever heard of the problem with LC 308 brass using the same powder charge associated with Win 308 brass. Do a search. A small tip - the difference between LC and Win 308 brass in case volume.
 
jlow said:
Have you ever heard of the problem with LC 308 brass using the same powder charge associated with Win 308 brass. Do a search. A small tip - the difference between LC and Win 308 brass in case volume.

I understand that LC 308 brass is thicker than Win 308 brass thereby having less case volume and as a result it is recommended that powder charges be reduced for the LC brass by about a grain or so as compared to the Win brass. But beyond that I do not get your point, please explain.
 
The point is case weigh reflects case volume and in turn affect chamber pressure. The weight variation we are seeing with Lapua brass is significant.

For example, in my personal experience, LC brass can weigh in at around 177 grains and Win brass around 157 grain which is 20 grain difference. Now even with a small sample size of Lapua brass I am seeing as much as 7 grain variation, so perhaps not enough to worry about overcharge problem if I am not close to max but one would think there would be significant difference in chamber pressure, MV, and POI? Surely the LC/Win is not an order of magnitude different (more like 2x) but if you have to worry about over pressure with that magnitude difference, it is not unreasonable to expect an effect on paper. A test at 10x higher excess volume variation would surely be dangerous and un-necessary?
 
I understand, good explanation. I understand that case weight is an indicator of case volume and for the same powder charge is an indicator of pressure. There appears to be two issues, one is accuracy and one is pressure; my comments were focused on the effect of case volume variation on accuracy. My suggestion for a test of the effect of case volume variation on accuracy did not assume that the test would exceed safe pressure. Such a test would have to choose a load that was safe for both the smaller volume and the higher volume. I only chose the ten percent difference in volume to make the two data points significantly different so as to amplify the effect of case volume on accuracy and would not need to be fine tuned loads. This would not be a load specific accuracy test but a test to help understand the effect of case volume variations on accuracy. But as you say there is also a pressure issue, a safety issue, if the case volume variation is significant and someone is at or over max with their load. I have made several observations that folks are reporting using loads on this forum that are over max as stated in the loading manuals. This is one more example of the risk in doing that. Now all that being said, I agree that 7 grains variation in Lapua brass seems high since my experience is one or two grains. It would be interesting to know what that 7 grains variation has to do with accuracy and even more important to pressure, safety, for someone who is already over max load. Thank you for your thoughtful response.
 
Here is a good article on subject under section 8:

http://www.6mmbr.com/jgcaseprep.html

I have seen a sudden variation is Lapua cases (.308 LRP) as well. The last lot was the worst I have had yet with regards to case weight variation.
 
T-REX - Not a problem. I too am not focusing on pressure in terms of safety (for this discussion) even though that can happen. My own interest is the same as yours i.e. effect on precision. I understand that you are looking for a significant difference in volume so that any effect would be obvious – that is usually a good way to go and certainly better than slight differences in volume which would lead to slight differences in MV which would be hard to detect. I think what concern me was your suggestion of using “an order of magnitude (ten times) the case volume variation that we normally experience and see if we could see an effect from that”.

So let me give you some numbers. For example, a 7 grain difference in brass weight I have found is equivalent to about 53 uL volume of water or 0.053mL (based on density of brass). Case volume of a 171 grain case is about 3.74 mL so the 50 uL is about a 1.4% difference in volume which BTW is pretty huge since a 1.4% of a 44 grain charge would be 0.6 grains. So a 10x change in case volume seen in the above would be 70 grain difference or 500 uL volume which would be a 14% difference in case volume – that is substantial considering that an LC brass only varies 20 grain from Win brass and nowhere close to 70 grains.

However, the fact is regardless of the above, the data is already there with the difference between LC and Win brass i.e. even with 20 grain difference in case weight, there is enough pressure difference to affect MV. The fact is precision (ability to hit in the same area, not necessarily the X which would be accuracy) is affected by pressure. At the extreme, pressure is certainly a safety issue, but at normal operating safe level, pressure affects MV, and inconsistent pressure gives inconsistent MV, and inconsistent MV is what cause vertical stringing of POI i.e. precision.

Tempest – Interesting that they think that they don’t like to use case lot greater than 1 grain in weight… There is one error in that write up. It says “A case that is heavier than the other indicates less interior volume, and the pressure will be greater than for those cases that weight less”. A heavier case does have less interior volume but the pressure is actually greater for those cases that weight MORE. More weight=less volume=more pressure.
 
How does this variation of weight and volume come into play with the 100-200 yd benchrest crew? I thought that it only mattered in long range as to the vertical problem. Please take my question as a chance to learn something I thought didn't matter in short range.
 
As far as I know, it does not matter for the 100-200 yard benchrest crew i.e. it does not matter in short range. I am only interested in shooting long range.
 
Tempest said:
Here is a good article on subject under section 8:

http://www.6mmbr.com/jgcaseprep.html

I have seen a sudden variation is Lapua cases (.308 LRP) as well. The last lot was the worst I have had yet with regards to case weight variation.
But they have missed the big problem The total head of the brass is machined difference n the total head diameter how wide and deep the grove is the inside primer pocket all makes a difference in total weight.
By checking the internal volume of the case is the only thing that matters. If a case weighs 1 gr different in total weight but has the same internal volume I will shoot the same speed.
Next week I will be doing another tuner test. I will take 10 cases with each having the same internal volume capacity. An another 10 with the same total weight but internal volume different. I will show the target and crony difference.
I have done it many of times but don't have the target or crony results.
I'm just trying to help not get in a argument.
The internal case volume is what pressure is based off of . When you change seating depth of a bullet you change the total internal capacity of the case.
Send me a pm with your phone number I will try to explain it better Larry
 
jonbearman said:
How does this variation of weight and volume come into play with the 100-200 yd benchrest crew? I thought that it only mattered in long range as to the vertical problem. Please take my question as a chance to learn something I thought didn't matter in short range.
Hi when you shoot bench rest .Normally you start with 15 to 20 new cases .By match time you have 8 or 10 that shoots the same. You have sorted them by how they shoot.
Take the number of the cases you started with and your best 5 check the internal capacity . I would like to know the results.
Larry
 
jlow said:
So let me give you some numbers. For example, a 7 grain difference in brass weight I have found is equivalent to about 53 uL volume of water or 0.053mL (based on density of brass). Case volume of a 171 grain case is about 3.74 mL so the 50 uL is about a 1.4% difference in volume which BTW is pretty huge since a 1.4% of a 44 grain charge would be 0.6 grains. So a 10x change in case volume seen in the above would be 70 grain difference or 500 uL volume which would be a 14% difference in case volume – that is substantial considering that an LC brass only varies 20 grain from Win brass and nowhere close to 70 grains.

I did some basic work with weight/volume/velocity about 10 years ago, and I found a correlation of 10gr of case weight to be equal to 1 grain of powder for equal velocities. I was working with 308/175 SMKs at the time.

I don't know if the same ratio holds for smaller cartridges.
 
CatShooter said:
jlow said:
So let me give you some numbers. For example, a 7 grain difference in brass weight I have found is equivalent to about 53 uL volume of water or 0.053mL (based on density of brass). Case volume of a 171 grain case is about 3.74 mL so the 50 uL is about a 1.4% difference in volume which BTW is pretty huge since a 1.4% of a 44 grain charge would be 0.6 grains. So a 10x change in case volume seen in the above would be 70 grain difference or 500 uL volume which would be a 14% difference in case volume – that is substantial considering that an LC brass only varies 20 grain from Win brass and nowhere close to 70 grains.

I did some basic work with weight/volume/velocity about 10 years ago, and I found a correlation of 10gr of case weight to be equal to 1 grain of powder for equal velocities. I was working with 308/175 SMKs at the time.

I don't know if the same ratio holds for smaller cartridges.

This is good information. It appears that you could extrapolate this information into the velocity difference for one grain of powder and have an estimate of the effect of 10 grains of case weight variation on the target. The only problem that I see is that it was pointed out that case weight is not the issue, the issue is with the volume variation that may result from case weight variation. What do you think, can we get to an estimate of the effect of case weight variation on what it is worth on the target?
 
T-REX said:
CatShooter said:
jlow said:
So let me give you some numbers. For example, a 7 grain difference in brass weight I have found is equivalent to about 53 uL volume of water or 0.053mL (based on density of brass). Case volume of a 171 grain case is about 3.74 mL so the 50 uL is about a 1.4% difference in volume which BTW is pretty huge since a 1.4% of a 44 grain charge would be 0.6 grains. So a 10x change in case volume seen in the above would be 70 grain difference or 500 uL volume which would be a 14% difference in case volume – that is substantial considering that an LC brass only varies 20 grain from Win brass and nowhere close to 70 grains.

I did some basic work with weight/volume/velocity about 10 years ago, and I found a correlation of 10gr of case weight to be equal to 1 grain of powder for equal velocities. I was working with 308/175 SMKs at the time.

I don't know if the same ratio holds for smaller cartridges.

This is good information. It appears that you could extrapolate this information into the velocity difference for one grain of powder and have an estimate of the effect of 10 grains of case weight variation on the target. The only problem that I see is that it was pointed out that case weight is not the issue, the issue is with the volume variation that may result from case weight variation. What do you think, can we get to an estimate of the effect of case weight variation on what it is worth on the target?

Case weight IS volume. What ever variation there is in the extractor grove is negligible.

I measured a bunch and the extractor grove was +/- 2 thou max, and most were half of that. Figure out what the weight is of a band of brass that is 2 thou thick, 30 thou wide, and 3/4" long,and you have...

... nothing worth the time.

There is more variation in the meniscus if you fill cases with water and weigh them.
 
I used to be EXTREMELY anal. I weighed and sorted everything, brass, bullets and yes, including primers! I sorted bullets, I had plastic fishing lure boxes filled with sorted bullets stacked upon each other. I worried and fretted and worried some more. It was an obsession.
Then two things happened that made me re-evaluate everything I did. The first was that I shot my best ever score benchrest match using the culled Lapua brass that I rejected and used only for practice that and bullets that were pulled from other practice brass. Reason being I had nothing loaded and wanted to shoot the next day. Shot a 250-24X. Having done that I used Lapua 6BR brass right out of the box and Berger 105 VLD's right out of the box to shoot a range record Ground Hog match score or 148-7X.
Oh, I still weigh every powder charge and practice good reloading techniques but don't bother with a lot of fretting over things that don't seem to matter much. If you are happy doing all that stuff, great, and I won't say it won't make a miniscule difference, but as long as we have environmental and human factors to contend with in the big picture.....well........ I spend a lot less time fretting and more time enjoying shooting.

Danny
 
I personally believe that it's possible that weight differences can indicate volume differences but not necessarily so. Correlation is not causation.

If you want to sort by case volume, you need to measure case volume, not weight. If somebody has conclusive evidence to the contrary, I'd be glad to be proven wrong; weighing a large number of cases is relatively easy, measuring volume on same is not.

Not to say there isn't value in weight sorting as a means to find truly irregular cases. I just weight sorted 200 pieces of Lapua 6.5x47 brass. Out of 200 there were exactly 5 cases that were more than +/- .5 grain off the mean.

-nosualc
 
Here is the data from 16 pieces of new Lapua 308 brass empty weight vs. internal rubbing alcohol weight. Corrolation coefficient =0.8398.
 

Attachments

  • Volume.jpg
    Volume.jpg
    15.7 KB · Views: 65

Upgrades & Donations

This Forum's expenses are primarily paid by member contributions. You can upgrade your Forum membership in seconds. Gold and Silver members get unlimited FREE classifieds for one year. Gold members can upload custom avatars.


Click Upgrade Membership Button ABOVE to get Gold or Silver Status.

You can also donate any amount, large or small, with the button below. Include your Forum Name in the PayPal Notes field.


To DONATE by CHECK, or make a recurring donation, CLICK HERE to learn how.

Forum statistics

Threads
166,791
Messages
2,224,021
Members
79,861
Latest member
srak
Back
Top