Case capacity

[Nature Boy]

I'm making some 308 this morning and have a question about the correlation between the effect of different case capacities on an accurate load. For instance:

Known accurate load using Fed 7.62 headstamp case with a H2O capacity of 54.9g vs a LC 14 headstamp with a 54.0g capacity, or 1.6% less volume.

Is the 1.6% difference linear and should I reduce my charge from 39.7g to 39.0 to achieve the same performance, or are there just too many other variables that make this one not relevant?

 

[Reloadron]

The general rule is when using military brass reduce charges by 1 or 2 grains when starting. Some people do not reduce their start loads at all figuring start is low enough. I am in the latter group and have never had a problem with my .308 Win loads in my military brass. My last batches were LC 13 and WCC 08. Since I don't know where your 39.7 grains figures into the Minimum/Maximum for your powder and bullet I can't make a suggestion but if the 39.7 grains is on the low side towards Ninimim I would not worry about it.

Ron

 

[Nature Boy]

Thanks Ron. I'm well below max. My question was more pertaining to the difference in results between two loads where everything is equal except case capacity. And if there is a noticeable difference, do you just compensate in a 1:1 ratio capacity/powder? I'm working off of a good, known to be accurate load.

Does that make sense or do I need another cup of coffee?

 

[Reloadron]

Thanks Ron. I'm well below max. My question was more pertaining to the difference in results between two loads where everything is equal except case capacity. And if there is a noticeable difference, do you just compensate in a 1:1 ratio capacity/powder? I'm working off of a good, known to be accurate load.

Does that make sense or do I need another cup of coffee?

Depends on where it puts me on the load but generally for what you have I would not worry about it since you are on the low side. As you have guessed this is not a very exacting science. Most of the hand loading manuals or cartridge reloading manuals I have sitting here don't mention the differences we encounter in brass, not that I see at first glance anyway. For what you are looking at, again I would not worry about it. That being just my opinion so hopefully others will weigh in with their thoughts.

Ron

 

[Bart B.]

Those who have made transducer pressure tests with case weight or capacity versus charge weights have ended up with the following:

For each 10 grains of case weight change, change the charge weight by 7/10ths grain. Up for lighter ones, down for heavier ones.

Case capacity varies a bit with case shape even if all cases weigh the same.

I've seen no difference in accuracy across .308 Win cases weighing 150 to 190 grains as long as the charge weight is somewhat less for the heavier ones. And neck grip on the bullets is the same across all so the force needed to push the bullet out is reasonably consistent.

As most folks shoot only a few shots per test group. a wide spread in results will vary across people shooting the same load and rifle. It's not a really big issue unless you shoot your stuff into less than 1/3 MOA at short range all the time.

 

[Nature Boy]

I loaded some LC cases and some Hornady cases with 37.1g of H4894 and got the following averages.

Hornady- 2368fps, LC- 2418fps

That's a 2% increase in velocity resulting from 5% less capacity in the LC case capacity.

What does that mean? I guess it means that the effect of the case's capacity to powder charge is not linear, or 1:1, and if you have a good load with one type, you still have to work up a new load with a change in case capacity. I was hoping there would be a mathematical shortcut

Or maybe I just need to break down and get QuickLoad

 

[VoodooMountain]

Theoretically you will get the same performance if you have the same pressure as that particular pressure on load 1 as you have discovered what the gun likes.

The 1.6% difference is not linear, however and there are other variables. Not to mention an accuracy node encompasses a small range of pressures so you don't have to duplicate the exact pressure which is good because it is an impossible task with even the most stringent reloading practices due to variances beyond your control.

In other words I would drop 1-2 grains on that load and work up. You will likely find the sweet spot at about where you are now.

 

[Reloadron]

I loaded some LC cases and some Hornady cases with 37.1g of H4894 and got the following averages.

Hornady- 2368fps, LC- 2418fps

That's a 2% increase in velocity resulting from 5% less capacity in the LC case capacity.

What does that mean? I guess it means that the effect of the case's capacity to powder charge is not linear, or 1:1, and if you have a good load with one type, you still have to work up a new load with a change in case capacity. I was hoping there would be a mathematical shortcut

Or maybe I just need to break down and get QuickLoad

No Shortcut.

As VooDooMountain mentions there are variables beyond the case capacity or volume. Neck tension, primer to primer inconsistencies and other variables. Even over the chronograph, was the rifle identically anchored or held down and supported?

Best you can do is work up a load that shoots well and then if it shoots well over a wide range of brass be happy, very happy.

Ron

 

[Nature Boy]

Thanks guys, that's what I'm looking for, I'll keep working on it

 

[RichCotte]

Thanks for the info

I was just about to ask a similar question. I'm working up a load in LC 7.62 brass that I've converted to 6.5 Creedmoor.

 

[TimSr]

I've found that no matter how good the formula and the calculations, they never seem to match reality. If you start low, and work up, you rarely fail, and if you change brass, start low and work up again. Nothing is ever linear.

 

[Gtscotty]

I don't know of a proven way to calculate exactly how much to adjust a load to ensure you stay in an accuracy node between two different case capacities. I would think the best you could really do is guess at a small adjustment, then work up and see if it shoots as well as the known load. For what it's worth though, 0.9 gr isn't that much of a difference in case capacity... If you're on a good node with one case i bet you would still be on with that amount larger/smaller case. At least that's been my experience with one of my good loads in .308 Winchester, FC, and Hornady brass. Generally though, i just try to get a decent amount of the same head stamp so it's not as much of an issue.

 

[Nature Boy]

Well I had a thread I started earlier trying to figure out what changed with a proven load that went from 0.5 MOA to 3-4 MOA. The only thing I could figure was a change in case brand/capacity (and powder/primer lots).

Just when I get cocky and think I got this whole reloading thing nailed, I'm proven wrong.

One of the regulars here posted recently to the effect that he's new to reloading and only been doing it for 30 years. There's a lot of wisdom in than statement.

 

[LiveLife]

There's a lot of experience and knowledge on THR.

Compared to what they know and have experienced, I am just another reloading newbie for rifle calibers and I have been reloading for like 25 years.

Thankfully, most will steer us in the right direction from the start. Not sure who pointed out the internal case volume difference between commercial and military .308 cases (with the benefit of buying LC headstamp cases with crimped primer pockets ensuring they are true once-fired) but that sure helped with my load development as I would still be sorting brass by weight and likely wondering what was happening to my groups using different internal case volume brass.

 

[Gtscotty]

Well I had a thread I started earlier trying to figure out what changed with a proven load that went from 0.5 MOA to 3-4 MOA. The only thing I could figure was a change in case brand/capacity (and powder/primer lots).

Just when I get cocky and think I got this whole reloading thing nailed, I'm proven wrong.

One of the regulars here posted recently to the effect that he's new to reloading and only been doing it for 30 years. There's a lot of wisdom in than statement.

Did you get that load reeled back in once you switched back to the original cases?

 

[Nature Boy]

Not yet Scotty. I have to process some more brass. I was hoping to get to it this past weekend but ran out of time. I'll definately know the answer this weekend. It's got to be the brass

 

[macgrumpy]

No, case weight is not linearly related to case volume.

This article (http://www.thefirearmsforum.com/attachments/reloading-pdf.45471/) is an analysis that proves there is no direct correlation between case mass and volume. The chart below comes from that article, notice that cases that have over 2 grains of difference had the same volume (55.2gr of water).

You can also prove it to yourself by experimenting with powder charge weights and the resulting velocities. Using a known case volume you use a couple of different powder charge weights and relate them to their velocities (a ratio of so many grains of powder for a certain change muzzle velocity). Now you can attempt to estimate the required powder charge to obtain a specific muzzle velocity with a case of a different volume.

Yes, you do reduce your charge but no there is no way of accurately estimating the required change. As most people recommend, reduce the load by at least 1 grain and then experiment. This is where you absolutely need a chronograph, it's the only way to verify that you are getting the same muzzle velocity. And yes, I check each now lot of bullets and cases. I just loaded some Lapua cases for my M1A. I prepped the older cases in minute detail and verified that all of the dimensions were very consistent. I didn't prep the newer cases (different lot) in as detailed manor...the result was that the newer brass required over a grain less powder to get the same velocity average but my extreme spread and standard deviation numbers were a lot worse.