Question on shoulder growth and headspace?


April 20, 2008, 10:15 AM

I'm not sure I'm using the proper terminology here so bear with me please...

I'm working on loads for M1 Garand. 45 gr IMR 4895 with 168 gr SMK.

I trimmed and sized once fired cases of the same headstamp in a Dillion 1200 size/trim unit. The cases fit the case guage perfectly and I established a zero point using a Redding dial comparator. Then I fired the rounds and remeasured the cases. The case shoulders had blown forward by 0.003" to 0.007"

1) Is this a normal growth range?

2) Why did they not all end up with the same growth?

3) If I'm setting the bullet ogive at 0.010" off the rifling, how much does the forward growth affect accuracy?

Thanks for your assistance,

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Grandpa Shooter
April 20, 2008, 11:47 AM
Growing brass is indicative of a chamber being not very tight. When you fire the round and the brass heats up and expands the brass moves forward into the front of the chamber. When it cools, newer more flexible brass contracts slightly. Old, more brittle brass does not contract as much.

The trick is to only neck size your brass after using it the first time so that you don't lose so much brass in the trimming phase. The only way I know of the find the amount of set back you want from the lands is to make up a dummy round which is longer than what you believe the chamber to be and gently chamber it. Look at the shoulder (ogive) of the bullet to check for contact marks. If they are non existant your bullet is too far back in the mouth of the case. If the marks are deep in the bullet you are too far forward. Look for tiny marks and then back off .001 for overall case and bullet length.

Working your brass too much will greatly shorten the number of times you can reuse the cases. By neck sizing only you can use the brass many more times.

I hope this makes sense to you. If not, someone else will come along with different phrasing.


April 20, 2008, 12:34 PM
You must full length size your brass for the Garand, and for all auto rifles, or they won't feed properly. With that said, every M1 Garand I've owned, and that's 9 of them, has had what could be called a "generous" chamber. They were made that way on purpose, so that under combat conditions, they will feed just about any ammunition fed them.

Every one of my Garands has had a chamber that measures approximately .007"-.008" over SAAMI minimum. I currently still have 5 Garands, and they all fall into that range in the Precision Mic.

If you're setting your case shoulders back to SAAMI minimum, you're going to experience case head separations and very short case life. You should set up your sizing die to just set the shoulder back about .002"-.003" from what they come out of the chamber at. Since you're getting .007" out of some of the brass, then I'd set up the die to set the shoulder back to .005" over SAAMI minimum. If the rounds chamber easily, then that's where I would leave it, unless your rifle gives you rounds that are longer than the .007" measurement.

The reason you're getting the different measurements is a combination of the elasticity of the brass being used, and how much it's been work hardened by firing/loading/firing/loading.

You'll get your best indication of the size of the chamber with new brass. Measure 5 rounds and get the average and then set up your sizing die accordingly.

Setting the headspace back to zero in the sizing die is one of the most common mistakes in loading for the Garand. It works for new brass, because it will stretch enough to fill the chamber, but on subsequent loadings, the brass will give out and leave the body of the case stuck in the chamber. You can prevent this by setting up your sizing die properly.

Hope this helps.


April 20, 2008, 04:37 PM
The bolt face moves under different pressure levels with each firing of an auto loader. A fixed bolt face as in a bold action does not move/unlock as the pressure builds. This can give different reading. Always FLRS for an auto.

April 20, 2008, 04:43 PM
Follow Freds advice and you will be safer and get better brass life. You will also get better accuracy as well, usually.

Grampa Shooters advise about new springy vs older work hardened brass is right on the money as well.

The work hardened brass will stop bouncing back at all eventually, but it is usually just about at the end of its useful life by then, unless you anneal it.

Oh yea, if you set up your sizer with older stiff brass, you will push the shoulder back farther than you want with new soft brass. Use Freds advise here as well. Set up your sizer with new brass.

April 20, 2008, 07:15 PM
Thank you gentlemen!

I will retry the experiment with new brass and report back.


April 20, 2008, 07:16 PM
When a round is fired in an auto-loader the brass expands to fit the chamber the same as in a bolt action. The difference is that the brass may still be under a slight bit of pressure when it is extracted. This tiny bit of pressure can cause the fired brass to be longer and also fatter than the actual chamber dimensions. This oversizing of the brass is not always consistant due to many factors such as barrel heat, powder charge and powder burn rate. Here is an article worth reading on loading for service rifles.

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