These are ten samples of case head to shoulder measurements of virgin WW Super .300 Win Mag brass. All of this brass was ran through a FL sizing die before loading, even though it is brand new. I do not trust almost all manufacturers to produce brass that is guaranteed to easily go into the chamber. Anyway, look at the measurements of brand new brass.

2.246"
2.249"
2.252
2.247"
2.245"
2.244"
2.248"
2.248"
2.244"
2.248"

A total variation of .008".

I got the rifle and load to shoot sub-half MOA anyway. But that is another conversation on seating depth, powder choice, and powder charge testing. This is for a Weatherby MK V.

Now look at the datum line (where the body turns the corner to the shoulder). Time and time again, the datum like becomes sharper, after one firing. This is fire formed brass now. The brass, under pressure of firing, was forced to completely fill the chamber, and take on the shape of THIS SPECIFIC chamber. More on that later.



I put ten pieces of once fired brass back in the calipers with a Hornady head space gauge still attached.



Fired brass, case head to shoulder measurements, for ten pieces of brass.

2.269"
2.269"
2.270"
2.270"
2.270"
2.269"
2.269"
2.269"
2.269"
2.269"

A total variation of .001". That is eight times better than the virgin brass!

The three pieces that measured 2.270" could have been flow back on the primers, I didn't look. I would call fired length 2.269". When the time comes to size this brass, trial and error has to happen. It is time consuming, but worth it. I would clean and lube the brass, place it in the shell holder, raise the ram, screw the Full Length sizing die down until I could not turn it down anymore, raise the ram, give the die an eighth turn, size the brass, then measure to the shoulder again. What I want to size to is 2.267" or 2.268". I will have to keep trying until I get the sizing die to create that measurement. This would be a .001" to .002" shoulder bump. Let's go back and look at an average measurement for the virgin brass. That is 2.247". Take fired length 2.269", subtract average virgin length 2.247", and the shoulder flowed .022". That is a significant move. Fired brass now has more powder capacity. But, in this case, I would not do a second load development, though I could.

What you are getting by only driving the shoulder back .001" to .002" is three things'

1. The brass (not neck sized) is guaranteed to cycle into a clean chamber, fire, and cycle out of the chamber. It is reliable cycling brass

2. We keep almost all that brass movement. What that does is let's the powder charge use all of its' energy to drive bullet, instead of flow brass. Second loaded brass, sized this way, on the same lot of powder, and the same charge more often than not creates more velocity. And we are not talking about an Ackley Improved. .300 Win Mag virgin brass, going in a .300 Win Mag chamber.

3. We are not over-working the brass. Firing the brass, it goes forward one to two thousandths, sizing the brass, it goes backward one to two thousandths. After two firings is when I clean and anneal. It has work hardened some, and I want to preserve the life of it, and maintain the elasticity of the neck. Primer pockets getting loose is why brass is tossed. Once I got into my two firings, anneal cycle, I have never had a case neck split. I got 7 firings on Hornady 6.5 Creedmoor brass. My Nosler 7mm-08 brass has six firings.

So, we talked about sizing for THIS PARTICULAR chamber. I opened a friend's box of fired brass, and measured his case head to shoulder, 2.275" for ten pieces. A six thousandth difference. When I need to size his brass, I need to adjust the sizing die again, to only size his shoulder back .001" to .002", which would be 2.273" or 2.274". That is a Tikka .300 Win Mag. Two mass production rifles, from two different quality manufacturers, in two countries, and we have a .006" difference in fired brass length. Treat each chamber, and the brass that belongs to it as individuals.

Excellent write up! I literally just went through the same process this week, and until I did so I had no idea of the variation in virgin brass. Once I measured and recorded the variation, I was actually surprised at the accuracy I obtained prior to fire-forming the brass.

My results before and after were very similar to yours, and in the same caliber, in fact.

I have compared new brass to fired brass a couple of times (without the detailed measurement that you did) and found the fired brass to be a little faster and a little more accurate than the new brass.

Yes, once fired, resized brass tends to shoot a little bit tighter.

It's obvious why.

Nice write up

Nice write up fireman. I went through the same exercise some years ago with a new batch of 7RM brass. I was surprised to see how much headspace new brass has and how much minimally bumping the shoulder cut down on trim.

I started reloading with factory fired ammo and then moved onto new brass. From your (FJG) recommendation, I started the habit of fireforming. Just anecdotal, but it does feel like I get better accuracy out of the later firings than my initial load. Funny thing is that I was really playing with reloading procedures so I basically did the exact same method twice or finding the best charge weight and both sets had the same charge weight yet firings 2, 3 and 4 just felt like it performed better.

Btw, this was Lapua brass so the headspace variation was good compared to 0.008", not 0.001" good. My is issue was that some case mouths looked messed up so I fl resized all of them. My new starline and winchester 308 brass was as bad in terms of headspace. However, my ADG came nicely packed with foam so all the case mouths looked perfect and the headspace measured well. I loaded these straight up and was super impressed while currently on fire forming this brass.

Good stuff, sir

That boy good.

Great info.

Good post Jason. Probably one of the most over looked areas of reloading.

Thank you sir.

Maybe it belongs in the sticky up yonder?

It does indeed. Good post sir.

I was fixin to ask.

Its there now.

He’s already hard to deal with. This will make it impossible.

It's too late for me.

Ask Judd