Rifle Brass Sorting Experiment - Part One

[ammolytics]

Hey everyone!

I published the first part of an ongoing experiment which explores the relationship between the weight and volume of rifle brass.

https://blog.ammolytics.com/2020-01-08/brass-sorting-part-one.html

The results were pretty interesting, and I was surprised by a few of the findings. I hope that it's high quality enough to justify the time you spend reading it, and how much time it took me to create it.

I'd love to hear your feedback and answer any questions!

[Duluth88]

That is a lot of data, look forward to digging into it.

Sent from my SM-N950U using Tapatalk

[JackEllis]

I’m not a long-range or serious competitive shooter but I am interested I. The practical implications of confirming or debunking “conventional wisdom” (aka “old wives tales”). So for what little it’s worth, here’s my take on your paper, which I thought was very well done and thoroughly enjoyed reading.

The Quickload simulations suggest every one percent variation in case volume results in a one half percent change in muzzle velocity. The range of volume variations in your sample is about a half percent after one firing and one quarter of a percent after four firings. In your sample, that suggests the variation in MV due to variations in case volume starts at about 14 FPS after one firing and drops to 7 FPS after four firings, which tends to confirm the conventional wisdom about fire forming.

Using your reference load and a ballistic calculator, I ran a couple of calculations assuming a 1.75 high zero at 100 yards to see how the POI varies at 500 and 1000 yards with a Hornady 140 grain ELD match bullet at a starting MV of 2710 FPS. At 500 the drop is 44.5 inches and at 1000 it’s 302.3 inches. At an MV of 2717 the drops are 44.2 and 300.4. At 2724 the drops are 43.9 and 298.5.

Realizing that case volume is only one source of variation in competitive shooting and that variations can be cumulative as well as offsetting, It seems to me your research supports a conclusion that buying high quality brass makes more sense than sorting cases by weight, which confirms the results of the precision shooter poll.

[divingin]

Nice response, Jack. Appreciate someone doing the math (especially since it means I don't have to...)

As to the OP, I'm just getting to your report, and looking forward to seeing what you find.

[JackEllis]

Nice response, Jack. Appreciate someone doing the math (especially since it means I don't have to...)

As to the OP, I'm just getting to your report, and looking forward to seeing what you find.

[J-cat]

So what the OP is claiming is that a 185gr Hornady case has the same internal volume as a 155gr Hornady case??

[JackEllis]

So what the OP is claiming is that a 185gr Hornady case has the same internal volume as a 155gr Hornady case??

[J-cat]

I took away that he didn’t see any evidence of case weight affecting internal volume. With that said, Hornady makes 185gr cases, 165gr cases and 155gr cases for the 308. Is the OP’s position that there is no difference in internal volume in these cases?

[JackEllis]

[J-cat]

It may be news to you, but it’s real. The brass they sell as components weighs @165grs. Their loaded ammo brass varies from @155 to 185 grains depending on the line.

Every 5 years or so I see someone try to prove that brass weight is unrelated to internal volume. And they do experiments just like the OP. And then some people post about lawyers censoring load data, not knowing that manufacturers like Hornady have a variety of brass out there and their load data has to defer to the lowest capacity brass variant.

Bottom line is cartridge brass composition is close enough between brands that we can infer capacity from weight. Heavy brass cases have less capacity than lightweight cases. There’s nothing that’s going to change that.

[JackEllis]

Well, I learned something new, sort of. This post https://thefiringline.com/forums/showthread.php?t=549685from The Firing Line supports the idea that .308 case weights vary significantly from one manufacturer to the next, but it doesn’t necessarily demonstrate similar variations in case weights from the same manufacturer. Three different types of cases from Lake City have surprisingly similar weights.

But in the ammolytics paper, the author is using brass from a single manufacturer and it is perfectly reasonable for the volume differences in brass fired once through his rifle to be relatively insensitive to case weight since the cases have expanded to fit the chamber dimensions while case weights remain unchanged.

The author acknowledges that he probably should have tallied case weights and volumes for his unfired brass, in which case we might be having a different discussion.

One other fact to keep in mind is that brass is roughly 8 times more dense than water so if my math is correct, it takes an 8 grain different in case weight to yield a one grain difference in case volume as measured by the water filling method. A 35 grain variation in case weight translates to about a 4.5 grain difference in case volume assuming no dimensional differences in parts of the case that do not affect volume, and an 8 grain difference in case weight translates to a 2% diffence in case volume for .308 brass.

[LynnJr]

http://www.benchrest.com/hoehn/html/ammo___brass.html

Your answer

[ammolytics]

I'm having a hard time understanding what you're trying to say, actually. Maybe you can clarify a few things?

Can you provide some reference to the claim that Hornady makes cases of different weights for the 308? Their website doesn't seem to indicate this, so some clarity may help.

As I wrote in the article, the cases I tested were 6.5mm Creedmoor from Peterson Cartridge. This served as my starting point for the research, but there's more to be done.

I didn't test Hornady cases or 308 cases and I haven't seen any company sell cases of vastly different weights like you stated, so I'm not sure where you get the idea that I would have claimed any such thing like you wrote here.

[ammolytics]

The author acknowledges that he probably should have tallied case weights and volumes for his unfired brass, in which case we might be having a different discussion.

One other fact to keep in mind is that brass is roughly 8 times more dense than water so if my math is correct, it takes an 8 grain different in case weight to yield a one grain difference in case volume as measured by the water filling method. A 35 grain variation in case weight translates to about a 4.5 grain difference in case volume assuming no dimensional differences in parts of the case that do not affect volume, and an 8 grain difference in case weight translates to a 2% diffence in case volume for .308 brass.

I did weigh the unfired brass and that data is in the article as well as the raw dataset. Measuring case volume of unfired brass, while interesting, is likely a useless dimension.

A 35gr difference in brass weight from case to case would be... alarming. I'm not following how the difference between brass and water density really matters for the measurements or comparison you're suggesting.

[J-cat]

I'm having a hard time understanding what you're trying to say, actually. Maybe you can clarify a few things?

Can you provide some reference to the claim that Hornady makes cases of different weights for the 308? Their website doesn't seem to indicate this, so some clarity may help.

Are you disputing my claim?

I have Hornady 308 brass that weighs 155, 165, and 185 grains.