Accuracy, relating to case sizing/shoulder set back?

[Shadow9mm]

Ok ill try and keep the premise simple. If you have 1 lot of brass, but they are sized differently how will this effect accuracy and load workup?

For example. Measuring from the base to a point on the shoulder. You have some cases sized at 1.180 and some sized at 1.188. The shoulder is set back 0.008 further on the first batch.

To my understanding this would reduce case capacity affecting the way the powder burns?

The case stretch during firing would also cause a change in case volume during firing reducing pressure and creating inconsistancy?


Reason i am asking. I have an overly roomy chamber in a rifle. I will be adjusting my dies for a 0.004 shoulder set back to try and increase consistency, improve brass life, and reduce trimming. I started out with factory ammo which i can size properly now. However i just received 250pcs of new starline brass which is coming in shorter. Im concerned that if i try and work up a load with this brass, and then size it to more appropriately fit the chamber, it may effect the load workup i did with the smaller brass, both in velocity and accuracy. I really dont want to burn the components, 250 rounds, trying to do fire forming.

Im fairly certain the size difference will have an effect. The question is, how much, and what if anything should i do about it.

[Wag]

It will have an effect but you likely won't know to what extent until you shoot it.

After shooting it, it will fire form to your rifle's chamber. From then on, neck size only and you should be good to go.

--Wag--

[Shadow9mm]

Originally Posted by Wag

It will have an effect but you likely won't know to what extent until you shoot it.

After shooting it, it will fire form to your rifle's chamber. From then on, neck size only and you should be good to go.

--Wag--

Working with a shoulder bump of 0.004 as its a semi auto gun. My concern is, Working loads up, the workup will be invalid as the brass will be sized differently after the first firing.

[darkgael]

take five new cases.....try fireforming them without full loads. Use :"fireform" loads

[Shadow9mm]

Originally Posted by darkgael

take five new cases.....try fireforming them without full loads. Use :"fireform" loads

See, thats why i come here. Sometimes you cant see the Forrest through the trees.

I will do 10rnds each. Shoot then prep 10 cases, and compare my prepped cases to the virgin brass. See if i get any accuracy difference or POI shift.

[Nathan]

I find that between a tight fit like 0.000” average bump to a 0.004” bump, I have seen groups tighten significantly in a contender. From 0.002” - 0.01” bump, my main issue I’ve seen is case life.

My preference is:
Bolt handle drops freely -0.001” for benchrest
Bolt handle drops freely -0.002” for hunting bolt guns
Fired -0.003” for semi-auto
Action closes freely -0.002” for hunting Contender

[stagpanther]

Working with a shoulder bump of 0.004 as its a semi auto gun. My concern is, Working loads up, the workup will be invalid as the brass will be sized differently after the first firing.

I generally do .0025 to .003 using fired brass as a yardstick with my ARC brass. I also look for signs of good gas sealing of the case body and neck to the chamber and signs of "flow creep" ahead of the case head. This is where a chrono can help a lot--it will probably show wide fluctuations in ED's and SD's if the cases are not conforming well and consistently to your chamber.

Instead of angst over the fired factory ammo--you could just bypass that altogether and simply pull the bullets and powder (stash them if you want) and start with just the brass--I do that frequently.

[HiBC]

Did you recently buy this barrel?
I might be sending fired cases to the barrelmaker.

For a 6mm ARC (I would guess) you want two things:

Accuracy

Case life.

A good chamber is key to both. I might ask about a replacement.

[Unclenick]

One of the features of the Ackley Improved chambers is the parent cartridges can be fired in them to form the brass. The brass starts expanding somewhere in the low thousands of psi and makes extra space in the chamber while the powder is still burning, so pressure comes down some over what you'd see in a normal chamber for it. That will affect barrel time and any effect on accuracy associated with it. And you can expect some pressure ring stretch to occur.

If you are using reduced loads to fireform cases, one thing you can do to minimize initial pressure ring stretch is to grease or oil the cases for firing. Unless there are a lot of severe tool marks in the chamber, that should let any stretching occur over more of the length of the case, reducing thinning. Motor oil with additives to dissolve carbon is what I would use, though I would have a face shield on during shooting to keep the oil mist from spraying into my face. Any action bedding you have may need to be cleaned of oil afterward. Just don't apply the oil in advance, as you don't want to allow time for it to perhaps penetrate and wet the primer.

[Shadow9mm]

Originally Posted by HiBC

Did you recently buy this barrel?
I might be sending fired cases to the barrelmaker.

For a 6mm ARC (I would guess) you want two things:

Accuracy

Case life.

A good chamber is key to both. I might ask about a replacement.

Im have reached out to l.e. wilson as fired cases wont go in their gage and they are supposed to. Going to be sending in the gage and some fired brass. also reached out to ballistics advantage have not heard back yet.

Right now i set my dies up for a 0.004 shoulder bump. My thinking is, having a roomy chamber will help with feeding and reliability since its a gas gun. Also the roomy chamber should ensure i dont run into any pressure issues even at max, kinda give me a little safety buffer.

[Unclenick]

On your calipers, how wide are your fired cases at the shoulder and at 0.2 inches from the base (or at the widest point in the wall, in which case, how far forward of the head is that wide point)?

[Shadow9mm]

Originally Posted by Unclenick

On your calipers, how wide are your fired cases at the shoulder and at 0.2 inches from the base (or at the widest point in the wall, in which case, how far forward of the head is that wide point)?

At the shoulder was 0.432, i measured that the other night. I also measured the base. I dont remember the measurement, but i do remember it was smaller than the 0.441 in the hornady illustration. Ill measure it again tonight.

[Unclenick]

If you look at the chamber dimensions in the SAAMI drawing for the 6mm ARC, and keep in mind the given diameters are minimums with a plus 0.002" tolerance, a SAAMI maximum chamber would be 0.4446" at 0.2" forward from the breech end, and the shoulder could be up to 0.4330" in diameter. You are within those diameters, so the Wilson should accept the case if it was properly cut for as-fired cases, which their instructions state they are. Sending it back to them with a fired case is about all you can do.

[FrankenMauser]

Full power "fire-forming" loads (put together with good components) are often some of the most consistent loads a rifle will ever fire.
The phenomenon has been discussed for nearly a hundred years.
The leading theory is that the extra case expansion helps "smooth" the pressure curve. But I don't know of anyone that has pressure test data for comparison.

I wasn't much of a believer until I saw it for myself. Now that I have seen the same thing happen with multiple rifles and cartridges over the years, I accuracy test every chamber that needs brass fire-formed -- during and after forming. It is a surprisingly frustrating experience; knowing what the rifle can do, but never being able to quite get back there unless only ever using 'virgin' brass.

I can't say that everyone in the industry is going to have the same opinion, but I don't believe you need to worry about the "reduced" case capacity (which is miniscule in your case, anyway). What you want to load for is the fire-formed case capacity.
What matters is the shape and size of the case at peak pressure (after it forms to the chamber), not the size and shape as-loaded.

If as-loaded mattered, Remington would have gone bankrupt 6+ decades earlier, with their ammo chronically well-under SAAMI spec.
I have a very tight .243 Win 'match' chamber that was cut to just barely chamber R-P brass. It is so far under SAAMI spec for a .243 chamber that the barrel is marked as a wildcat chamber. R-P is/was the only brass on the market that fit that chamber, because Remington tended to make things so far under SAAMI spec. There is no measurable change in the brass after firing, except in the neck. (I don't know what I might have to do now, if the new owners are making brass closer to spec. I guess I'll find out eventually.)

So my tight .243 is *not* one of the instances where fire-forming was ever a consideration.
But I bring it up because Remington ammo was notoriously well under SAAMI spec for decades, with far more expansion on initial firing than people fret and agonize over while theorycrafting on forums.
If 0.006" at the shoulder, and 0.006" in the body, and 0.004" at the base, and 0.007" in the neck were as big of a deal as it seems to be on the internet, Remington ammo would have blown up 50% of the guns in the US by now.

[Unclenick]

Once a fellow shooting by himself on the next range over came up to me with a 308 case with no neck, and asked me if this blown out curiosity was normal. He'd managed to fire the 308 in a 30-06 chamber. No damage to the gun, but I don't think the bullet had hit the 50-yard sighting target he'd put up. The brass held together. Not recommended for normal use, but good proof excess headspace is not a direct burst hazard.

Hatcher experimented with a chamber cut progressively deeper in a 30-06. IIRC, doing firing testing at every step in the cutting. IIRC, at something like 0.050" excess headspace he actually got an increase in velocity. I don't recall any explanation for that.

Anyway, excess headspace is not an existential threat to the rifle. The brass life will stink, though.

[stagpanther]

Anyway, excess headspace is not an existential threat to the rifle. The brass life will stink, though.

Except possibly in the rare event if the case head ends up with inadequate support (poor brass and chamber design)--which happened to some of the initial 350 legend AR roll-outs--I had one and wasn't happy when my rifle blew up.

[Unclenick]

That's never good. Unsupported chambers are always at risk with bad brass.

In most guns, a very long headspace just won't be used. Instead, the cartridge winds up headspacing by its rim, stopping on the extractor hook. This is common in 1911's, but the 45 ACP isn't running at a pressure that sticks the case to the chamber walls, so the whole case just backs up against the breech face. That's why 45 Auto never gets pressure ring stretching and thinning. For a rifle cartridge headspacing on the extractor, you should see both the shoulder blow forward and the pressure ring stretch. If there is a blowout or head separation at the pressure ring, that's bad brass, an overly fat chamber, a bad extractor hook design, or some combination of those. It wouldn't happen if things are made right.

[stagpanther]

That's never good. Unsupported chambers are always at risk with bad brass.

In most guns, a very long headspace just won't be used. Instead, the cartridge winds up headspacing by its rim, stopping on the extractor hook. This is common in 1911's, but the 45 ACP isn't running at a pressure that sticks the case to the chamber walls, so the whole case just backs up against the breech face. That's why 45 Auto never gets pressure ring stretching and thinning. For a rifle cartridge headspacing on the extractor, you should see both the shoulder blow forward and the pressure ring stretch. If there is a blowout or head separation at the pressure ring, that's bad brass, an overly fat chamber, a bad extractor hook design, or some combination of those. It wouldn't happen if things are made right.

Winchester's "Gen1" rushed release of chamber and cartridge specs resulted in the possibility of headspace tolerance stacking of up to .01" and still be within original specs as opposed to the more common .006 +/-. Even some go no/go gauges released encompassed the .01--with the sole exception of PTG's that stuck with .007" that I found early on. The brass has an unusually large rim groove; the top of the web consequently could be below the chamber breech face; especially if the chamber face is radiused (usually are). Being the knucklehead hobbyist that I am and thinking that I must have made a mistake--I called a prominent manufacturer of custom chambered ARs--he told me he found the same thing--and asked me if I'd be interested in taking all of his 350 legend ammo off his hands (which I did, at a price).

The result: KABOOM.

[badmatrix]

This is my first post. It is refreshing to see someone with the same issues i had with this six months ago and gave up. The numbers didn't add up and it was driving me nuts. I used a Hornady comparator or the Rcbs precision mic.

I saw a youtube video that put it all together.

With fire formed brass from .223, .308 or 6.5creedmoor, after annealing, I would measure a handful with the comparator to get an average.

With my RCBS sizing die, i raise the ram until it makes contact and resize on cartridge, this is my starting point. I will resize and then measure this cartridge and compare it to my average headspace when I started.

I will then screw in the sizing die little by little until it measures 0.002-0.003” "back" from my beginning average, lock it down and size away, taking the occasional reading.

Hope this helps. I will post the link to the video that put it all together for me.

https://youtu.be/QdNdTQsdOy0