6mm ARC Headspace measurement?

[KMW 1954]

I have a Savage 110 Switchback 6ARC coming and will be reloading for this and just getting started/ready.

Have about 300pcs of once fired brass that has been donated from a friend that doesn't reload. Have dies and the inserts for the Hornady comparator/headspace gauge. Have also downloaded the 6ARC SAAMI print for a start.

Now I was just checking some brass with the headspace comparator and spec lists 1.190 - 1.2001 as min and max in one spec yet the print itself has a dimension of 1.190" -.007" so is the range actually 1.183 - 1.200"?

I ask because a lot of this brass I am measuring at 1.180" which would appear to be short. Just found one at 1.1765". So how concerning is this?

 

[DHD]

It's not.

2 ways I would do this:
1- To get your base to shoulder measurement is to fire some brass from your rifle. You can bump the already fired brass given to you a couple thousandths and load it. Now get your measurement.

2- If you want to be more precise, remove your firing pin and ejector (depending on action) and size that already fired brass until the bolt will close without any resistance. I typically will check 5 or so pieces of brass before I call it good.

For my bolt actions, I feel this is a more precise measurement. Well, in truth, when my bolt drops on it's own using the above technique, I lock the die down and don't really need a measurement for that die and that gun/barrel as it is as good as it is going to get. I still measure the brass of course, but it is only to make my OCD feel better.

For an AR, I would bump the fired brass you have .003" and go to the range. When I got home I would have my fired brass to measure. I would still want to bump that shoulder .003" though.

Just what I do....

 

[KMW 1954]

I understand all that and is what I normally do only at this time I do not yet have the new rifle in my possession. Nor have I picked up any factory to use as a baseline. Yet again this brass measured the same way I normally check headspace as short as 1.1765" while SAAMI SPEC is 1.190" which is .0135" short. So why would I want to bump it back shorter?

 

[DHD]

I missed the part where you didn't yet have your rifle and can see what will fit.

 

[KMW 1954]

I missed the part where you didn't yet have your rifle and can see what will fit.

Yeap I seen that.

So maybe I'll just have to hold off until I have the rifle and can fire some fresh factory loads.

 

[Pres100]

I would just wait until you get the rifle, remove the ejector and firing pin and size the brass down to fit your chamber. That should give you the best starting point without overworking the brass. For there you can fire them a time or two in your rifle and remeasure for better dimensions.

 

[KMW 1954]

Normally I find no need to remove anything. Just fire 5-10 rounds of fresh factory from the gun and then measure the headspace.. Measuring the fired brass from my 223 is always within .001" of each other no matter how many times it was reloaded and fired..

Just hoping to get a jump start on this round to gain familiarity but it doesn't look like I can trust this brass to be able to do that. This brass is measuring all over the scale. So is this indicative of the Hornady brass or the rifle it was shot out of?

 

[Pres100]

Do you know for a fact that all of the brass was fired out of a single rifle and only once? WAs any of the brass random range pick up mixed in with what your friend gave you?

 

[KMW 1954]

I do have some odd range pickup brass but that has been kept separate from the rest. Also have 40pcs of Starline brass that was also given to me and it is much more consistent than the Hornady.

 

[Murray Brook]

The Hornady tool is just a comparator, it doesn't measure headspace. What it does is allow you to measure a fired round and you can then adjust your die to get the shoulder bump that you need. Just compares your fired case to the sized one

 

[KMW 1954]

The other night while bored watching the tube I sat and weighed 100pcs of the Hornady and the Starline.
The Hornady weighed from a low of 109.92gr to a high of 116.00gr, a spread of 60.8gr..
The Starline weighed 108.84 low to 109.98 high, a spread of only 1.19gr!

 

[KMW 1954]

The Hornady tool is just a comparator, it doesn't measure headspace. What it does is allow you to measure a fired round and you can then adjust your die to get the shoulder bump that you need. Just compares your fired case to the sized one

And a powder measure drop doesn't measure weight either, it measures volume. So?

 

[Murray Brook]

Then I ,guess I'm trying to say don't measure weight with a powder measure as I wouldn't trust a comparator when it comes to true headspace measurement..

 

[gunsandgunsmithing]

The Hornady tool is just a comparator, it doesn't measure headspace. What it does is allow you to measure a fired round and you can then adjust your die to get the shoulder bump that you need. Just compares your fired case to the sized one

Yes, like Murray says..the Hornady tool is not designed nor intended to give actual headspace measurements, but to give the difference between a fired and sized piece of brass, by using a consistent contact point. Other than an optical comparator, I'm not sure how would be best to measure actual headspace dimensions. I'm sure there's something, though.

 

[DaveTooley]

Yes, like Murray says..the Hornady tool is not designed nor intended to give actual headspace measurements, but to give the difference between a fired and sized piece of brass, by using a consistent contact point. Other than an optical comparator, I'm not sure how would be best to measure actual headspace dimensions. I'm sure there's something, though.

Take a ring gage the appropriate size, surface grind it to remove any flair at the mouth and you're good to go.
This is the first site I clicked on.

Cylindrical Ring Gages - Steel / Chrome

Ring Gage inspects the external dimensions of manufactured parts A Ring Gage can also be used to set an instrument, air or electronic An annular groove in the periphery of the NO GO ring distinguishes it from the GO ring Gagemakers tolerance will be applied minus on the GO and plus on the NO GO...

westportcorp.com

 

[DaveTooley]

I did something similar using neck sizing bushings. I did it so I could have an apples to apples comparison when talking to my customer about bullets. I think I started with 10 bushings and got 6 or 7 that measured the same after surface grinding them.

 

[KMW 1954]

Oly WoW, Where the blazes did this all come from?

I am fully aware that the Hornady tool is just that, a Tool, designed to Compare like objects. Now this has turned into a lesson in semantics! Taken completely out of context to boot!

There are any number of companies that offer Headspace gauges for setting headspace on a rifle and action, still every one of those has a tolerance to boot. Even the SAAMI drawings themselves list a tolerance.

I really didn't expect this simple question to get derailed as easily as it has. So far you all have discussed every point other than the one asked!

 

[Murray Brook]

Sorry if I derailed your thread. Most certainly wasn't on purpose. I think what I meant is that I wouldn't depend on the print specs when measuring with the Hornady tool.

 

[gunsandgunsmithing]

I have a Savage 110 Switchback 6ARC coming and will be reloading for this and just getting started/ready.

Have about 300pcs of once fired brass that has been donated from a friend that doesn't reload. Have dies and the inserts for the Hornady comparator/headspace gauge. Have also downloaded the 6ARC SAAMI print for a start.

Now I was just checking some brass with the headspace comparator and spec lists 1.190 - 1.2001 as min and max in one spec yet the print itself has a dimension of 1.190" -.007" so is the range actually 1.183 - 1.200"?

I ask because a lot of this brass I am measuring at 1.180" which would appear to be short. Just found one at 1.1765". So how concerning is this?

Oly WoW, Where the blazes did this all come from?

I am fully aware that the Hornady tool is just that, a Tool, designed to Compare like objects. Now this has turned into a lesson in semantics! Taken completely out of context to boot!

There are any number of companies that offer Headspace gauges for setting headspace on a rifle and action, still every one of those has a tolerance to boot. Even the SAAMI drawings themselves list a tolerance.

I really didn't expect this simple question to get derailed as easily as it has. So far you all have discussed every point other than the one asked!

The numbers you gave are meaningless because of the tool you used. That's why you got the replies you got. Everyone was trying to be politely helpful and got scolded for that.

To answer your question, I have to make assumptions but as you probably also know, new brass is very forgiving and can be fireformed to much greater amounts than you likely have, ie, a Dasher or brx. Once fireformed, the rest is up to you in maintaining proper shoulder bump based on the new dimensions for formed brass. It's common for new brass to be short. Your once fired brass will likely be just fine.

Also, you appear to be conflating cartridge drawings and chamber drawings.

https://saami.org/wp-content/uploads/2021/06/6mm-ARC-Public-Introduction-Rev-2020-07-29.pdf

 

[Eagle Six]

I really didn't expect this simple question to get derailed as easily as it has. So far you all have discussed every point other than the one asked!

You got your answer in the very first reply.....

You asked.....

So how concerning is this?

And @DHD answered....

It's not.

You are using a comparator gauge to compare various fired brass without having brass from the gun you do not have as yet. Once you have the rifle, have fired some loads you will have fired brass that you can compare to the donated brass and then make a decision what you want to do with the donated.