For those of you who measure such things, how much does the headspace grow from a new 300 WSM brass (WW in my case) after being once fired. Mine grows about .027 in and I was curious as to whether this is a normal amount.

I believe you're using headspace to mean the distance from the face of the case head to the datum line on the shoulder, right?

For comparison, if I size a piece of 308 Win. brass all the way to like new and shoot it in my deepest chamber the growth is .007" tops.

Yes, I'm measuring the distance from the face of the case head to the datum line on the shoulder. The rifle is a 2001 ULA, so the first year for the 300 WSM. It seems a bit generous, size wise, but I don't have any other 300 WSMs to compare it with. I set the shoulder back .004 after each firing, and so far, I've gone 5 reloads on the same brass.

I'm not a WSM loader so I don't know what's typical between new brass and first firing dimensions.

How many different batches of brass have you used to make this comparison? I know from experience when a new batch of 308 is short or long in this dimension.

I've measured three different bags of Winchester and two different Federal pieces of brass (one nickel plated) that were factory loaded (I obviously removed the bullet and powder). All were relatively short compared to my chamber and and grew the full .027 upon first firing.

I don't know. Someone with several WSM's needs to chime in.

I've had several and with new brass and its first firing, I usually got about .013" stretch. After it is fire formed and even FL sized I only saw about .003" growth after each firing... .027" sounds a little extreme to me...

You may just have a sloppy, or long chamber.

I"d consider neck sizing.

If you look at a SAAMI chamber and cartridge drawing it lists chamber headspace as min and max and this covers .010. And then between the GO and NO-GO gauge you have .004.

I collected milsurp rifles and the worst variation was on my Enfield rifles with .017. And the worst on a new rifle was new cases being .011 shorter than the chamber. Meaning the case was .009 shorter than the GO gauge and my chamber was in the middle of the GO and NO-GO gauge.

Bottom line and my guesses would be hopefully some how your measurements are way off. Or the worst news is your have sever lug setback and you need to have a gunsmith check the rifles headspace.

I do not know how old your rifle is or how long you have ben reloading for it and I hope its human error in measuring. BUT the short magnum cases had a bolt thrust problem. And this was made worst if any oil or grease was left in the chamber. And the headspace would grow very quickly if the chamber was not kept dry.

Part two

Below is what happens when you fire a commercial factory .303 cartridge in a fat and long military Enfield chamber. Meaning civilian cases are not made Ford Truck Tough like military brass is.



And I bought a RCBS case mastering gauge to measure case stretching in the base web area.



The above gauge is nothing more than a fancy and expensive bent paper clip that can measure brass thinning in .001 of an inch.

The factory loaded once fired Winchester .303 British case below stretched .009 on its first firing. And why they talk about fire forming cases to fit your chamber.



The correct term for how much the case is shorter than the rifles headspace setting is head clearance or the air space between the rear of the case and the bolt face.



And if you actually do have .027 head clearance on your rifle you have one very sick rifle. And you would be getting case head separations on the first or second firing.

Normal military headspace on the military Enfield was .064 minimum to .074 maximum and emergency wartime headspace was .084. meaning .020 head clearance. And on a rifle with a good venting system a casehead separation doesn't hurt anyone. But normaly a milsurp rifle with .027 excess headspace is called a "Wall Hanger"

Be safe and be carful.

Whoever seated that primer didn't do a good job.

Logger, do you mean new, never fired brass grows that much on it's first firing? I've seen more than one piece of new WW brass below SAAMI minimum over the years, but never that much.

I just measured several pieces of .300 wsm brass,

Factory new Nosler Custom brass unfired, 3.770,

Once fired brass from same factory box, 3.775,

Several different pieces with several different measurements, all show about .005-.006 growth.

Measurements include headspace gauge with caliper zeroed.

I'm not sure why you are measuring anything. You are sizing correctly. It doesn't matter what the measurement is unless the chamber is way out of whack. As long as the sized brass fits your chamber, load them and shoot.