measuring shoulders

[KMW 1954]

I now have a new used 6mmARC and as of yet I have zero experience with this cartridge. So to start the learning curve I purchased a box of Hornady Black 105gr. to use as a baseline.

Before doing anything else I took these factory loads and measured with my Hornady comparator both the CBTO and CBTD or shoulder measurement. Well the CBTO was a little sporadic but the base to shoulder was surprisingly consistent at 1.1825" to 1.1830". Again this was to just gain a reference point.

So yesterday I was finally able to fire some of these and today I remeasured the cases base to shoulder. These now measured consistently at 1.1870" and 1.1880".. Does this measurement look normal? A .004" to .005" stretch. Will these stretch again further if I reload and push the shoulders back to say 1.1860"? Trying to get an idea of how much they will grow before they are fully fire formed. I've never done this with new brass before.

 

[GLShooter]

Bump about 0.002 to 0.003 on you shoulders. Your numbers are where you want to be. Firing them two or three times is fine. On an AR you're kind of chasing your tail going for fully formed brass. The case stretch after firing really makes all you case growth inconsistent.

 

[KMW 1954]

Guess I should have added that this is a Savage bolt rifle that is going to be used for short bench shooting.

 

[GLShooter]

In that case I only bump 0.001. I shoot a 243 LBC that is the big brother to your ARC. I use mine in 500 yard F-Class. 8208 is your friend for powders BTW.

 

[KMW 1954]

Before this I was shooting a Model 12 Savage 223 with a 26" barrel and yes only bumping back .001"

Right now using Benchmark as 8208 is extinct in these parts.

 

[GLShooter]

That's another good choice and is easier to find.

 

[Murray Brook]

@KMW 1954
What I do is I take a virgin case and install a primer part way into the case and then chamber the round and close the bolt. I then measure the case with my Hornady comparator. I do this several times and take the average #. From that averaged number I will subtract the bump I'm looking for. That works good for me and lets me know what my chamber measures without several firings on one case. JMW

 

[KMW 1954]

Murry I have seen you post that before. Much of what I use is once fired brass I get from the range I work at. Advantage is I can see where it came from. I have learned, reloaders will protect their brass and guard it. The others will gladly give it away.

I have found that after the second time being reloaded it has become pretty stable out of my rifles. But I believe this gives me a pretty good baseline of where this rifle is.

 

[K22]

If I am intrepreting your post correctly, you're getting .004 to .005 linear expansion on the first firing. You asked if that is normal. It is for that lot of brass and your rifle. On the first firing, sometimes a few more, the case is forming to the chamber.

I would not bump the shoulder back since you are going to create an oversizing / overworking cycle. The case will keep trying to form to the chamber after you bump it. The only caveat is that if a sized case (zero bump) will not chamber or if excessive resistance is encountered. Then you will need to bump shoulder.

I have often found that virgin cases do not require a shoulder bump for several cycles. I suspect that this is due to the F/L also sizing the radial dimension of the case. For example, I have one rifle, 223 Rem bolt rifle that I have not had to bump the shoulder and it's on its 6th reload.

 

[KMW 1954]

If I am intrepreting your post correctly, you're getting .004 to .005 linear expansion on the first firing. You asked if that is normal. It is for that lot of brass and your rifle. On the first firing, sometimes a few more, the case is forming to the chamber.

I would not bump the shoulder back since you are going to create an oversizing / overworking cycle. The case will keep trying to form to the chamber after you bump it. The only caveat is that if a sized case (zero bump) will not chamber or if excessive resistance is encountered. Then you will need to bump shoulder.

I have often found that virgin cases do not require a shoulder bump for several cycles. I suspect that this is due to the F/L also sizing the radial dimension of the case. For example, I have one rifle, 223 Rem bolt rifle that I have not had to bump the shoulder and it's on its 6th reload.

K22 thanks!, that was my question. With zero experience with this chambering I have no way to judge if this is a normal amount, excessive amount or a conservative amount. Also with no experience I have nothing to judge how many it takes to settle into a consistent state. As I've learned what to expect with the 223 brass.

This that was just fired seemed to measure pretty consistent. So now I also have some loaded from donated once fired brass that measured all over, meaning .012" from shortest to longest, that I will now fire in this and see how close it comes to this 1.188" as I know much was shorter than this to begin with.

 

[K22]

K22 thanks!, that was my question. With zero experience with this chambering I have no way to judge if this is a normal amount, excessive amount or a conservative amount. Also with no experience I have nothing to judge how many it takes to settle into a consistent state. As I've learned what to expect with the 223 brass.

This that was just fired seemed to measure pretty consistent. So now I also have some loaded from donated once fired brass that measured all over, meaning .012" from shortest to longest, that I will now fire in this and see how close it comes to this 1.188" as I know much was shorter than this to begin with.

Even though in my younger days, I used range brass, I am not a fan of using range brass anymore from unknown sources fired in unknown rifle chambers especially for precision shooting.

However, I understand the situation today with scarcity of components. I would just be careful using it, making sure it chambers and extract properly plus monitoring cases for excessive stretch (incipient head separation). Also inspect the rims for excessive wear or tearing. If you encounter any loose primer pocket, do not use those cases since more than likely then have been subjected to excessive pressure resulting excessive case head expansion.

While a strongly believe in measuring case head space using a bump gauge, I always spot check in the rifle chamber until a confidence level is achieved with the sizing set up. It is the ultimate gauge in my opinion.

The very best results precision shooting wise that I have achieved is with virgin cases dedicated to a specific rifle, rotating their use, and F/L sizing for optimum fit in the rifle chamber.

 

[KMW 1954]

Even though in my younger days, I used range brass, I am not a fan of using range brass anymore from unknown sources fired in unknown rifle chambers especially for precision shooting.

The very best results precision shooting wise that I have achieved is with virgin cases dedicated to a specific rifle, rotating their use, and F/L sizing for optimum fit in the rifle chamber.

The used brass is from people I know that do not reload. I always inspect all brass very closely before use.

Thankfully now I do have 100pcs of new Starline brass on it's way. should have been here last week but for some reason the PO has it stalled in St Louis for over a week..

 

[searcher]

Bump about 0.002 to 0.003 on you shoulders. Your numbers are where you want to be. Firing them two or three times is fine. On an AR you're kind of chasing your tail going for fully formed brass. The case stretch after firing really makes all you case growth inconsistent.

Because of that inconsistent A/R shoulder situation, I set my bump to no more than .002" from my longest case. I record that measurement and, after firing, any case longer than that becomes my new long case to re-set bump .002" off that. No sense in automatically bumping all cases as GLShooter alluded to.

 

[KMW 1954]

Not loading for an AR Platform so things should prove to be much more consistent and manageable. Also from past experience with a bolt 223 by the second reloading my cases all stabilized and were very consistent when measured using the comparator.

Now with the firing of these factory loads in this rifle I should have a pretty good baseline to compare the once fired brass I received. Then once the new Starline brass finally arrives I can proceed with that too.

 

[DHD]

I have found Starline 6mm ARC brass to be pretty consistent out of the bag in OAL and weight. I bought 100 rounds of Hornady factory ammo and checked the weights on some of the brass and wasn't impressed. I really just wanted the factory ammo so I could check the measurements on some once fired brass (bought more than I needed). I was measuring the factory ammo and got the same numbers as you were KMW 1954 using the Hornady .350" bushing.

Even though my 6mm ARC is an AR, my measurements with once fired using the .350" bushing is 1.887", so very close to your measurements. I ended up setting my Forster FL die to 1.885". The sized brass chambers in my Proof barrel fine and as of right now it's the only 6mm ARC barrel I have so I'm fine with only a .002" bump.

Not that it applies to the OP, but because I was curious I pulled some of the 105 HPBT factory ammo. I found 30.5 to 30.8 grains of powder that looks very similar to LeverRevolution but who knows. I measured 30.6 grains of the powder back into the neck sized cases and put a 95 SMK back .025" off. Wow. I'm not going to do anymore than the 20 but it was accurate. Looking at Hornady loading data there is a good chance it is LeverRevolution (or Hornady's version of it) and loaded to their maximum data for the 105 HPBT. I'm not surprised the 95 SMK shot well.

 

[AlNyhus]

KMW1954: When measuring the base to shoulder datum on a fired case, remove the fired primer first. The fired primer can add a few thou to your figures.

 

[KMW 1954]

KMW1954: When measuring the base to shoulder datum on a fired case, remove the fired primer first. The fired primer can add a few thou to your figures.

Every time, first thing is a wash in hot dish soap with Lemi-shine then remove primers, them check case measurements. Been doing that with 223 for 3.5 years, it's a habit now.

 

[Olde Man]

Remove the primer from the case. Remove the firing pin from the bolt. Chamber the fired case. If you feel any resistance in the last part of the bolt closure bump the shoulder back 0015 to .002 and rechamber. If the case is not felt on bolt closing start there. Record the size. If the sizing it is too tight as you size and fire your cases you will get an occasional case that is tighter than you would like. Change your bump another .0015 or .002. What you are messing with is the spring back or yield of the brass. It is hard to get consistancy when you are only bumping .001 or when shooting a light load and that brass is not expanded to the max chamber size.

 

[Webster]

I now have a new used 6mmARC and as of yet I have zero experience with this cartridge. So to start the learning curve I purchased a box of Hornady Black 105gr. to use as a baseline.

Before doing anything else I took these factory loads and measured with my Hornady comparator both the CBTO and CBTD or shoulder measurement. Well the CBTO was a little sporadic but the base to shoulder was surprisingly consistent at 1.1825" to 1.1830". Again this was to just gain a reference point.

So yesterday I was finally able to fire some of these and today I remeasured the cases base to shoulder. These now measured consistently at 1.1870" and 1.1880".. Does this measurement look normal? A .004" to .005" stretch. Will these stretch again further if I reload and push the shoulders back to say 1.1860"? Trying to get an idea of how much they will grow before they are fully fire formed. I've never done this with new brass before.

It doesn't matter what the new factory base to shoulder is. You only need the measurement on a cartridge fired in your rifle. The factory overall length is set so it will fit in any rifles magazine. The factory base to shoulder is always excessive. Your making it more complcated than necessary. I make the over all length 0.010" short of touch for all my reloads. Many ranges require single load for safety reasons. I single load in the field for GH hunting. In 50 years I don't think I ever tried see if they would magazine feed.

1. Buy a bump gauge and caliper which you may already have. Measure base to should on a fired case. As long as it chambers easily I wouldn't bump the shoulder. When it gets a little tight to close the bolt, that's the number you bump from.

 

[okie]

Guffey must be gone forever....????
I miss the Hilarity!