Using the Hornady headspace tool...

[You-Two]

I bought the Hornady headspace measuring tool with the intent of sizing my rifle cartridges to the specific chambers. My first try has led to some questions...mainly concerning the measurements I took compared to the SAAMI specs. The rifle is a new Marlin XL-7 in .30-06 and the brass is once fired Rem (from that gun). I zero'ed the caliper at 2.0" with the tool installed and double checked I was using the correct diameter gauge.

SAAMI spec for .30-06 is: 2.0487-2.0587

My average reading for 30 cartridges was: 2.043 (lowest was 2.042 & highest 2.044). The average for the remaining unfired Rem I have was 2.040.

I also tried the RCBS Precision Mic and it averaged -.001.

Does this mean I have an undersized chamber? Or am I doing something wrong? Also, I did the same thing with three rifles in .223 (2 x ARs and 1 x Savage 12fcv) with the same type of results. All the measurements averaged below the min specified by SAAMI.

Any insight would be appreciated!
Thanks...

 

[JimKirk]

My question for you ...is what you gonna do now? According to your measurements you rifle is under size... so what do you do? Trade rifles, ream out the chamber or you can do what the most of us do ... reload to fit YOUR chamber. That is the most practical thing to do. Size your brass to fit your chamber and shoot the heck out of it! What does it really matter if it is under or over, unless it is so far out of specs. that impossible to load for... then I would worry... until then ....size to fit and shoot.

Jimmy K

 

[243winxb]

Comparator

Does this mean I have an undersized chamber? Or am I doing something wrong?

Spring back of the brass may account for some of your short measurement.. The .0057" difference between the shortest Sammi you listed seems excessive. I would guess that this tool is not to check for correct Sammi headspace. The tool is called a Comparator . So, compare fired brass to sized brass and forget about Sammi. To get a true reading of a chambers headspace by measuring brass , you need to neck size only, till the brass will no longer chamber easly, (bolt action only) this method will give you the most correct size of your chamber. See photos of my home made Comparator at photobucket link below. This is how i see it, i could be wrong. Click for larger photo >

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[Walkalong]

Yep, a comparator, not a headspace tool, and yes, a once fired piece of brass will not be the size of the chamber due to spring back. Hammer it a few times with full loads in the same chamber until it gets tight as 243winxb posted, and it will be close. To get a real measurement on a chamber make a cerrosafe casting of the chamber.

On once fired brass you can set the sizer up to get the shoulder back to where it is as fired, and it will chamber fine with very little stretching of the case. ( I say "get the shoulder back" because as the case is being sized the diameter is reduced pushing the shoulder forward at first until you get the die screwed down enough to contact the shoulder) As the brass gets work hardened, and without adjusting the sizer, the shoulder will end up farther forward due to work hardening and spring back after sizing, but it should still chamber fine since we were not at max for your chamber anyway.

 

[kelbro]

I'm not sure from your post exactly what you are measuring and I have already had one cup of coffee .

If it is your chamber, then you really need a cast. Cerrosafe will provide that. There are also chamber plugs that you buy or make to determine the length of your chamber.

If you are looking at OAL, that's different and will vary from bullet to bullet.

I use the comparator to determine where my ogive need to be in order to achieve xx.xx off the lands. SAAMI does not come into play with that measurement.

 

[rg1]

Yes the Hornady tool is just a comparator which will let you measure brass fired in your rifle and adjust your sizing die to push the shoulders of the case back a measured amount. The RCBS Precision Mic is close but in my experience can vary a thousandth or two. Here's a couple articles on both:
http://www.inlandshooters.net/index.php?contentid=99
http://www.realguns.com/archives/035.htm

 

[You-Two]

My question for you ...is what you gonna do now? According to your measurements you rifle is under size... so what do you do? Trade rifles, ream out the chamber or you can do what the most of us do ... reload to fit YOUR chamber. That is the most practical thing to do. Size your brass to fit your chamber and shoot the heck out of it! What does it really matter if it is under or over, unless it is so far out of specs. that impossible to load for... then I would worry... until then ....size to fit and shoot.

Jimmy K

Jimmy K,

Yes, that is what I intend to do. It seems to chamber factory ammo just fine. I was just a bit confused by the difference between my measurements and the SAAMI specs.

Beau

 

[You-Two]

Thanks for the input everyone!

I was under the false impression I could measure the actual chamber headspace from the once-fired brass (in a bolt action anyways) using this tool. I have read in several places it is good to reduce brass stretch by sizing just under the actual chamber, like around -.001 or so for a bolt action.

In this case I would want to size down to 2.042 based on my fired measurement of 2.043. That is a relative number and not the actual chamber number.

Hopefully I'm on the right track now...thanks again! Great articles by the way rg1.

Beau

 

[kelbro]

Now I gotcha. You are using the case length gauge. I was thinking about their other gauge that you use to determine OAL for touching the lands.

 

[Faret]

If you look at the end of the gauge it is chamfered and that might be where some of the measurement is getting thrown off too compared to sammi specs.

 

[243winxb]

For a bolt action, i just put a .004" shim between the shell holder and FL die when adjusting it. Size 3 brass, then see if the bolt will close on them. You may need to fire and size the brass at this setting 3 times or more, till they grow to there full cartridge maximum.