Headspace Micrometer Measurements & Precision Reloading

[MaverickNH]

So I'm beginning to work through the EddyCoyle MassReloading process learnt but never practiced, and thought I'd check in.

I took my AR15 Service Rifle to the range and popped off 20rd of ammo - sadly, I thought I had some decent ammo in the bag but only had some 1987 M1A3 South African 556x45 surplus. Silly me.

Once home, I ran a Forster 223rem series though a RCBS Precision Mic 223rem and made a plot. It took a while to get a reproducible technique down, so I restarted once I had a system: I would put in gage, ammo or brass, screw down just to pre-snug, unscrew 2 turns rattle, re-screw, and then unscrew, turn case/gage/round a bit, re-screw, etc. I recorded the lowest measurement that appeared twice in 10 tries. Yeah - a lot of work! No marks left on brass, so I think I wasn't cranking down too tight.

The plot has a an messy area, just around the 1.4696 FIELD Gage - I measured the 1.4689/1.4696/1.4606 series several times, but get about the same measurements. I'll have to check with Forster. Maybe they grind a lot of GO/NO GO/FIELD units and those differ from the others in the series?



Then I measured 20 fired and unfired rounds. The unfired ammo was highly variable at 1.4630 +/- 70% with fired brass tighter at 1.4681 +/- 7%, for an average expansion of 0.005.

Is my process looking OK so far? I'm assuming that the nasty ammo, once fired, is a decent measure of chamber length. It's crap brass, rather than my planned Lake City 1x-fired brass for precision reloading. Is firing "rebound" variation a factor I need to consider between crap brass and LC?

 

[asp]

If you want to get down to the nitty gritty details, the amount it deforms between new and once fired can affect neck tension on the next reload. I've never trusted ammo out of any box to be truly matched to my gun. I haven't conducted a scientific study, but it would be interesting to get a batch of brass that deforms .010 and another that you know will deform .005, reload them identically and shoot groups of 5 or 10 at 100 yards to see how accuracy differences measure. The other probably more repeatable solution is to anneal the brass.

 

[EddieCoyle]

The fired brass can't be any longer than your chamber, but it can end up shorter.

Set your sizing die for the longest fired case -0.003"

Also - the GO, NO/GO, and Field gages have nothing to do with ammo.

 

[Steve Marshall]

My favorite anecdotals about fired brass almost always center on .303 Enfields. They have, as a group, the loosest chambers I've ever encountered. My favorite one loved "neck sized" brass, as in the die was backed out until only about 1/8" of the neck was being sized. I had one that was so tight, short and/or crooked, it wouldn't allow a fired case back in. Sadly I never went after the reason.

Anyway, back on topic, for an individual reloader, the raison detre for case gages of any sort is to get you in the ballpark. However you measure your brass the final arbiter is how your firearm "likes" it. Go back to the Enfield- it headspaces on the rim. If you call it a day, you'll get head seperations within a few reloads. Size the brass so it contacts the shoulder in the chamber and you'll see a case that typically won't look like factory ammunition.

So, OP- fit your ammunition to the rifle. If uncomfortable with that, do what EC said, LFC -.003".

 

[MaverickNH]

The fired brass can't be any longer than your chamber, but it can end up shorter.

Set your sizing die for the longest fired case -0.003"

Also - the GO, NO/GO, and Field gages have nothing to do with ammo.

Just using the Forster gages to "calibrate" my RCBS Precision Mic. It worked out pretty well, with 10.3 units on the Mic dial corresponding to 0.010 difference in gage length (based on the trendline).

I'm getting it - as there was a lot of spread in the unfired ammo headspace, some might not have expanded to chamber size, so I want to look at the longest data and size -0.003 for semiauto rifle. So looking at charts below, I have maximum length fired rounds at Mic reading 4.3 (calculated 1.4687). If I'm understanding you correctly, I'd be sizing to 1.4687-0.003=1.4685, or Mic reading calculated 1.5, or maybe 1-1/3 to 1-2/3 by eyeball.

Measuring with the RCBS Precision Mic seems to be a touchy thing. I do a finger rub on the casehead to feel for burrs or crimp extrusions, then blow in Mic base and drop in, put a bit of downward pressure and twist in case there is bit or burr I missed, then spin top down to stop point, then give a slight snug, observe reading, spin up Mic top, rattle case, spin down and snug, observe reading. Then unscrew top, turn case, and repeat observations. I can see quite a variation, but figure that brass is soft and might trap a slightly slanted resting position, so that the lowest number is my best number. That, and the shoulder is likely not perfectly concentric nor the rim perfectly square. Yeah, I tend to overthink things...

 

[mac1911]

The fired brass can't be any longer than your chamber, but it can end up shorter.

Set your sizing die for the longest fired case -0.003"

Also - the GO, NO/GO, and Field gages have nothing to do with ammo.

If all 223 brass was at the max of 1.4666" and chambers cut perfectly @ 1.4636 our brass would be resized back to .003" when chambered.....
For service rifle I took my average of my fired cases in the current batch off 500pc
Took the 3 longest fired cases and did as EC said above. Set my die up to show .003-005" back.
Not a single issue since. Resize is smoother and easier, trimming is minimal. I measure after 2 loads. If they measure over max I run the lot through the trimmer.
I use both the rcbs and Wilson gauges.....