Brass Sizing Discussion

[alamo5000]

I am about to start load development of my 6mm ARC. I have over 100 something pieces of brass that I shot from factory ammo.

My question, or rather the discussion I would like to have is about resizing brass.

An unfired round straight from Hornady measured 29.2mm at the shoulder. After firing my brass measures 29.4mm. In inches that's .00787 inches difference in fired vs unfired.

Keep in mind that I might not have used the exact right comparator for the measurement so I might have been slightly up on the shoulder a little bit, but the measurement from both fired and unfired were done using the same tool. I did it to know what the difference is, or in other words how much my cases grow in my chamber.

In the image below I am focusing on (roughly) the dimension highlighted in red.

Now, with that said I have never really been a guy that chases lands or any of that but I do find the 'jam' point as Erik Cortina called it in his video(s). On that note there are two ways to get to that jam point. You can either have brass that is shorter or longer (or possibly inconsistent), or you can simply seat the bullet longer or shorter to compensate.

In your opinion, given the above mentioned numbers, what would the possible 'ideal' amount to size my brass back to from it's fired state?

 

[alamo5000]

Side note: I am a long time reloader. By long time I have been doing it for probably nearly 20ish years or more. That said almost ALL of my previous reloading was done for what most would consider 'bulk' ammo. I can and do reload pistol ammo. I have reloaded 5.56 and other sundry calibers. What I haven't done though is really gone down the rabbit hole of genuine precision, at least not a lot.

That being said I've developed loads that work better than I can shoot (if you are measuring simply by a 100 yard group)...but with the 6 ARC I am going for something new (to me). Even though I have been doing it a while I am still trying to learn something new here.

 

[KnowNothing256]

Ok, so I wanna make sure I understand properly. You're saying you wanna use the Eric Cortina method of determining jam, which (as I understand it) has to do with determining/setting the seating depth of the bullet, as measured from the cartridge base to ogive (of the bullet), "CBTO." However, you're asking about sizing brass, specifically how far to reduce the headspace, which most of us around here call "bumping the shoulder," since the cartridge headspace is measured from a specific diameter datum line on the shoulder of rimless bottleneck cases like the 6 ARC.

I've marked up your pic; "jam" has to do with the light blue line (roughly), while sizing brass, specifically bumping shoulders, has to do with the green line. Looks like the 6 ARC cartridge uses a 0.350" datum line on the shoulder, which I've boxed in green.

To answer the question I believe you're asking, how far to bump the shoulder, 0.002" to 0.003" in a bolt gun is usually perfectly satisfactory.

As a side note, you state that you determined how much cases grow in your chamber, but it would be more accurate to say you determined how much factory cases grow in your chamber. If you only bump the shoulder 0.002" then you can expect those resized cases to grow by about the same amount of 0.002", since they started out longer than the factory brass.

Does that answer your question?

 

[alamo5000]

Ok, so I wanna make sure I understand properly. You're saying you wanna use the Eric Cortina method of determining jam, which (as I understand it) has to do with determining/setting the seating depth of the bullet, as measured from the cartridge base to ogive (of the bullet), "CBTO."

Kind of but not really. I have been doing something very very similar to what he does to find jam, but I did it forever, and long before I ever knew who he was. Not saying I am some fantastic shooter but I found on my own that it works. I am not questioning anything at all about finding the jam or any of that. I am trying to figure out the 'optimal' length of my brass for a long range/precision type of application.

I did however forget to mention that I am asking about a gas gun. (at least in this case)

However, you're asking about sizing brass, specifically how far to reduce the headspace, which most of us around here call "bumping the shoulder," since the cartridge headspace is measured from a specific diameter datum line on the shoulder of rimless bottleneck cases like the 6 ARC.

Yes. From my measurements given above what would be my best 'target length' of my brass?

When loading 5.56 or whatever other blasting ammo 'if it fits it ships'...but in this case before I resize a single piece of brass I am using this as a learning tool for myself.

I've marked up your pic; "jam" has to do with the light blue line (roughly), while sizing brass, specifically bumping shoulders, has to do with the green line. Looks like the 6 ARC cartridge uses a 0.350" datum line on the shoulder, which I've boxed in green.

To answer the question I believe you're asking, how far to bump the shoulder, 0.002" to 0.003" in a bolt gun is usually perfectly satisfactory.

I failed to mention I am shooting a gas gun. Probably not optimal for other shooters here but I am working with what I got. I am quite confident that there is a lot of crossover though.

.002 inches = .05 mm. That's barely touching it but hey, if the formula works it works. At very least it gets me started with being a lot more meticulous in this particular reloading process.

As a side note, you state that you determined how much cases grow in your chamber, but it would be more accurate to say you determined how much factory cases grow in your chamber. If you only bump the shoulder 0.002" then you can expect those resized cases to grow by about the same amount of 0.002", since they started out longer than the factory brass.

Yes, that is 100% correct. I was trying to determine how much my brass grew from a factory standpoint vs my specific chamber. That is exactly correct.

Does that answer your question?

It certainly helps quite a bit. That's for sure. A follow up question would be, let's say brass is sized really short. Meaning in a hypothetical situation brass got basically way over sized. For the time being leave the gas vs bolt thing out of it... how will having short brass impact things? What would the real world results be from having short brass? Let's assume they are all the same length per the original question but they were just short... would that potentially make groups open up? What are some of the other symptoms that could happen there?

Keep in mind that when I previously created 'precision' ammo for hunting loads or whatever I basically did a whole lot of the things that are discussed here, I just didn't know how to put 2+2 together in expressing that. On previous times I would have the rifle in my hands and tinker with everything until I got the group size to be satisfactory for that given rifle. Once the dies were set up they stayed set up. I would save a few 'example rounds' and write the charge weight down and go from there. Before, the charge weights were about the only thing that was written down.

I would just use the exact same headstamp of brass and go for it.

It would not be wrong to say that I basically did 'redneck science' to get what I was after...this time though I am trying to be extremely deliberate in what I am doing.

 

[KnowNothing256]

To answer your follow-up question first: really REALLY short brass is unsafe, because it stretches during firing too much and leads to case separation, typically near the web. I don't think that's what you're asking though, I think you mean excessive but still safe sizing. If so, then the consequences will be potentially (but not necessarily) reduced accuracy and guaranteed over-working of the brass. Over-working of the brass will lead to shorter case life and also frequent trimming, since when you push the brass too far down it has to go somewhere, and that somewhere is the neck length. There's no good reason to do this unless your driving goal is cross-functionality across platforms, such as factory ammo (which is why you saw 0.008" growth in your Hornady brass); if you're looking for precision and multiple firings from a piece of brass, you wanna minimize sizing.

I don't own or operate a gas gun, so I'll defer to others on how much to bump shoulders on gas-gun brass, but I'll say that I've seen recommendations on here to target 0.004" bump for ARs. It's possible to achieve this by backing off on the sizing die, but this is not nearly as consistent or controlled as using Redding Competition Shellholders. These shellholders aren't cheap, but you get a set that lets you adjust shoulder bump in 0.002" increments, and they make the process so consistent that you can just take note of which shellholder provided the desired bump on a fired shoulder, then use it every time. I just plug in the right shellholder when I start my reloading process and use it for sizing, neck expansion (mandrel), and bullet seating, it just lives on the press. I highly recommend these, although it's true that they aren't necessary.

Other than my deferring to those who load for gas guns, is that the info you were looking for?

 

[alamo5000]

This video below goes over the topic kind of nicely. The differences are that I am shooting a gas gun (however I think the same principles still apply). What I want to do though is make notes (has not been my habit so far, at least nothing as specific as I am talking about).... and trying from the get go to set up my dies correctly without nearly as much trial and error as in the past.

The only difference is, is I am focusing ONLY on setting up my dies, and ONLY on consistently sizing my brass, and to be able to get into the 21st Century in my methodology to accomplish a goal.

 

[alamo5000]

To answer your follow-up question first: really REALLY short brass is unsafe, because it stretches during firing too much and leads to case separation, typically near the web. I don't think that's what you're asking though, I think you mean excessive but still safe sizing.

Exactly. Yes. We are reading the same thing.

If so, then the consequences will be potentially (but not necessarily) reduced accuracy and guaranteed over-working of the brass. Over-working of the brass will lead to shorter case life and also frequent trimming, since when you push the brass too far down it has to go somewhere, and that somewhere is the neck length. There's no good reason to do this unless your driving goal is cross-functionality across platforms, such as factory ammo (which is why you saw 0.008" growth in your Hornady brass); if you're looking for precision and multiple firings from a piece of brass, you wanna minimize sizing.

This is valuable information. Thank you. I am not interested in making my ammo work in somebody else's rifle.

That said I saw that dude Eagle Eye Shooting on YouTube bitching that factory ammo won't shoot well in his rifle. Part of me doesn't really jive with his ways, but I did have a hypothesis that factory ammo is sized a little bit short and could possibly cause accuracy issues. If nothing else even some inconsistent results.

After measuring my brass I have been wondering about the growth (from factory) to fired lengths and if it's normal or excessive. I don't recall the last time I've had to deal with factory ammo. I never buy factory ammo. I think the last time I shot factory ammo was around 2014 or something like that. I wouldn't be shooting any now if I could find brass to buy. But things being what they are I am having to harvest brass from factory loads which has led to some of these questions.

I don't own or operate a gas gun, so I'll defer to others on how much to bump shoulders on gas-gun brass, but I'll say that I've seen recommendations on here to target 0.004" bump for ARs. It's possible to achieve this by backing off on the sizing die, but this is not nearly as consistent or controlled as using Redding Competition Shellholders. These shellholders aren't cheap, but you get a set that lets you adjust shoulder bump in 0.002" increments, and they make the process so consistent that you can just take note of which shellholder provided the desired bump on a fired shoulder, then use it every time. I just plug in the right shellholder when I start my reloading process and use it for sizing, neck expansion (mandrel), and bullet seating, it just lives on the press. I highly recommend these, although it's true that they aren't necessary.

I am aware of the shellholders. I am not opposed to buying a set or two of them. In recent years I have upgraded my press, dies, and other equipment so I am not at all opposed to a shell holder set. Those various upgrades are another factor that leads me to questions. Instead of redneck science I now have a lot more ability to measure what I am doing a lot better.

 

[KnowNothing256]

That video makes sense, and at least as it pertains to your question, I'd say it jives with one of the things I mentioned but didn't stress as much as Mr. Cortina did: accuracy is consistency. To that end, if you want to make your shoulder bump consistent, I recommend the Redding Competition Shellholders. Unless I'm mistaken, you'd want Set #12, since the 6 ARC base looks like it matches the 6.5 Grendel base of 0.441". To use these, you run the ram up, screw the sizing die all the way down until it stops against the shellholder, back off the ram, then turn the die in another 1/4 to 1/2 turn to ensure full positive contact with the top face of the shellholder.

Now, hopefully someone else on here will verify that 0.004" is the bump you want. Hope that provides the info you're after!

 

[DAMNKID]

If you brass growth is .078” you have bigger problems than wondering how much you should set your shoulder back.

 

[alamo5000]

To that end, if you want to make your shoulder bump consistent, I recommend the Redding Competition Shellholders. Unless I'm mistaken, you'd want Set #12, since the 6 ARC base looks like it matches the 6.5 Grendel base of 0.441". To use these, you run the ram up, screw the sizing die all the way down until it stops against the shellholder, back off the ram, then turn the die in another 1/4 to 1/2 turn to ensure full positive contact with the top face of the shellholder.

I am probably going to get some of those shellholders but damn. $70 per shot. That said I will probably end up with a couple of sets. The correct equipment makes life easier.

That being said I wonder if just adjusting the die in or out will be just as effective? I could see that if someone didn't have proper 'die control' that those shell holders might create a lot more 'moving parts'...

 

[Dthomas3523]

If you brass growth is .078” you have bigger problems than wondering how much you should set your shoulder back.

Pretty sure he left out a zero. .2mm is about .008

 

[alamo5000]

If you brass growth is .078” you have bigger problems than wondering how much you should set your shoulder back.

Typo was corrected.

 

[alamo5000]

Pretty sure he left out a zero. .2mm is about .008

This is correct.

.00787"

 

[alamo5000]

If you brass growth is .078” you have bigger problems than wondering how much you should set your shoulder back.

It was a typo... yes.

 

[DAMNKID]

Passed right by the metric reading that dthomas caught. But yes that makes a big difference from .078” to .0078” I would aim to set your shoulder back ~.003” should be sufficient. You can come close to that by running your die down slowly and taking measurements from base to datum.Once you have your desired set back “bump” check it with another fired case to verify your measurement adjust as needed.