Brass headspace shorter after firing

What's your shoulder diameter before and after?

I've seen it from time to time, depending on the chamber and the die, where (I think) the shoulder is blowing 'out' more than it's blowing 'forward', so the case doesn't appear to 'grow' because it's stretching out radially instead of axially.

Just a suggestion.

If you shoot/test someplace where loading at the range is convenient, then working just a few cases for several cycles while starting up a new rig is worth the trouble of hauling the gear out.

This way, you can learn the wants/needs of the chamber and the sizing recipe, learn what to expect for brass life, learn how the barrel responds to cleaning, get an idea of the speeds/bullets the rig likes, have the gun boresighted, all in half a morning.

As always, YMMV

Interesting! Today I was measuring headspace on different milsurplus 308 I ran through my Solus a few weeks ago and had the same result with 2010 Privi FMJ and 82 Malaysian ball. I’ve never had that happen either, didn’t even know it could happen. The Malaysian measured the same loaded and fired at 1.625 and the Privi was 1.6245 loaded and 1.623 fired. I only started paying attention to such things last year, so I don’t have much of a track record, but will be watching it in the future.

The other thing I found is that the difference in headspace between my fired brass and reloaded rounds in FGMM brass is .005”. I could dial it back a bit closer to .002”, but I use this same die to load for a M1A and AR10 so I’ll have to get measurements from those rifles before I tweak it. Might be time for the Redding shell holders that allow headspace adjustments so I don’t have to adjust the die for the different rifles.

JRBullock1987 said:

I shoot in my back yard basically. So that's not an issue. I've been shooting this rifpe/barrel for well over a year. That's what weird. I've had different brass headspace like some being fully fureformed to the chamber at 1.459" and some at like 1.456" that aren't quite fully expanded but never had headspace end up shorter than before I shot.

Click to expand...

Remember too... that the tool you are using to take that reading is only touching at a certain diameter on the shoulder and your tool probably has a little chamfer on it that makes the reality of where it touches some sort of complicated area contact.

ETA: It should be noted plainly, that we are telling you your results are not unusual and that it takes several cycles to fully form cases. Keep cycling a few without bump if they will chamber, and see where they end up. Sometimes the diameters become the challenge before the headspace does.

If you have the inclination and own a fine scale, take the water capacity readings of a few virgin cases versus some fully fire-formed sized cases as a reference baseline to compliment your dimensional inspections.

It is the change in that internal volume represented by the water weight change, that tells you as much or more since it includes the diameter effects.

You certainly need to pay attention to the fit of the brass for reliability of feeding and brass life, but if your process for prep is very consistent, then the effect on the target and MV stats will end up okay.

An AR is one of the more challenging examples for getting tight MV stats, so give yourself a chance to get there and don't freak if your SD isn't under 15 yet.

The more consistent those fire-formed cases are after your sizing process the better. And that internal volume is just another way of looking at the effects of the diameters at the same time as that shoulder datum. Those effects are combined when it comes to combustion and internal ballistics.

There are terms called entropy and enthalpy in the firing cycle, where in English that means there is wasted work expanding the case to the chamber, and heating the case. We just want those losses to be as identical as possible within the batch for every shot.

If we are using the goal of the standard highpower target for the AR Service Rifle, you would be surprised how much slop you can get away with up to the 300 yard line and still clean the target. But the slop ends there cause you don't get away with it for 600 or beyond.

So, two steps to the learning curve: first the brass must always fit within specs, but, step two... if your goal is to clean the 600 yard line then your prep process needs to be on point.

(Did anyone ever mention there are a significant number of Distinguished, HM, and MA level Service Rifle shooters who only shoot virgin brass at the 600? That is because their prep process isn't giving them the performance they need with their fired brass.)

Nothing wrong with learning close inspections of the cases and your prep process, but you can always skip to the chase by taking your best efforts with 20 shots slow fire at the 600 yard target and that will tell you if you need to go troubleshooting your loading process.

Keep at it and don't give up, you will get it there.

The case pressure was too low to really push back hard against the bolt face and force any longitudinal stretch up against the case shoulder.
But enough pressure to expand the brass diameter to release the bullet and mostly seal the chamber....resulting in slightly shorter headspace measurements.
Put more powder in the case to up the chamber pressure.

Did you really measure the headspace on every piece of brass before (or after) sizing?

If not, could some of them have been shorter to begin with?

This happens all the time, brass gets wider and shorter trying to fill the chamber. Easier to grow in diameter than length, path of least resistance . Layman's terms, obviously many variables involved .