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Info on headspacing

Wizard1183

Sergeant of the Hide
Full Member
Minuteman
Jul 14, 2018
244
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So In a semi auto AR10, fire formed rounds of 7.62x51 in chamber measures 1.635” on fired brass using hornady Headspace gauge.

308 brass fired measures 1.6295-1.632”

now I do have a go gauge of 1.633 though never actually used it in the barrel due to not being able to remove the extractor pin.

my question is is headspace of 1.624” oversized (full length sized) on 7.62 brass? And would ideal measurement be 1.6315-1.632” on 7.62 and 1.626-1.627” on 308 brass?
 
I like to go .003 to .004 under what the head space is. You want a little room, esp in a semi auto.
 
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Right. That’s what I’ve decided. However is 1.624” headspace too much?
Also I bought a Forster headspace go/no go, it doesn’t state which? But if I dump in a Lyman case gauge (I know they’re crap but it fits in it. Also I measured headspace in hornady comparator gauge and it’s 1.628” rather than 1.633” as stated on the go/no go Guage.
 
1. Can’t use fired case for hs measurement because AR stretches the brass.

2. AR bumps the shoulder on cases as the bolt goes into battery.

Size the cases in small increments until you can close the bolt on it with the upper off the lower. No, it’s not going to give you the perfect accurate number but it will be good enough for reloading.
 
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1. Can’t use fired case for hs measurement because AR stretches the brass.

2. AR bumps the shoulder on cases as the bolt goes into battery.

Size the cases in small increments until you can close the bolt on it with the upper off the lower. No, it’s not going to give you the perfect accurate number but it will be good enough for reloading.
Only way I can close that bolt is by letting it slam home. Extractor is still in and no way I can push it closed.
 
Right. That’s what I’ve decided. However is 1.624” headspace too much?
Also I bought a Forster headspace go/no go, it doesn’t state which? But if I dump in a Lyman case gauge (I know they’re crap but it fits in it. Also I measured headspace in hornady comparator gauge and it’s 1.628” rather than 1.633” as stated on the go/no go Guage.
The Hornady tool gives a reference measurement at the shoulder however the gauge itself is sized to the actual length of a hypothetical chamber at 1.633” and the measurement on the gauge reflects that.

Do you have an AR10 with a 308 win chamber or a 7.62 nato chamber?

If your 1.633 gauge (308 win) chambers completely in the rifle then I’d remove the gauge, stick it in your Hornady headspace gauge and zero the dial indicator. Then adjust your die until your sized brass is measuring - .004”. Or simply .004” less than 0 if using an analog dial indicator. That should put you at .004-.006 clearance which is fine.
 
The Hornady tool gives a reference measurement at the shoulder however the gauge itself is sized to the actual length of a hypothetical chamber at 1.633” and the measurement on the gauge reflects that.

Do you have an AR10 with a 308 win chamber or a 7.62 nato chamber?

If your 1.633 gauge (308 win) chambers completely in the rifle then I’d remove the gauge, stick it in your Hornady headspace gauge and zero the dial indicator. Then adjust your die until your sized brass is measuring - .004”. Or simply .004” less than 0 if using an analog dial indicator. That should put you at .004-.006 clearance which is fine.
7.62 NATO chamber. However it’s shot fed gold medal which is 308.
 
I can’t close bolt on the gauge with extractor pin still in. It’s hard as hell to try to close it. And as for removing it (extractor pin) off the bolt? That’s not going to happen.

I guess only option is to FLS a case, to say 1.634” or 1.635” and let slam home and see if it closes? Then subtract 003-004 from that if it closes?
 
For factory ammo, Id look for the 7.62 versions of FGMM as the brass is sized for 7.62 nato chambers - a lot less brass stretch.

Why can’t you disassemble the bolt?

Also, are you measuring your fired brass with the spent primer still in or did you remove the primer before measuring?
I can’t close bolt on the gauge with extractor pin still in. It’s hard as hell to try to close it. And as for removing it (extractor pin) off the bolt? That’s not going to happen.

I guess only option is to FLS a case, to say 1.634” or 1.635” and let slam home and see if it closes? Then subtract 003-004 from that if it closes?
 
For factory ammo, Id look for the 7.62 versions of FGMM as the brass is sized for 7.62 nato chambers - a lot less brass stretch.

Why can’t you disassemble the bolt?

Also, are you measuring your fired brass with the spent primer still in or did you remove the primer before measuring?
No primer or I take a razor blade and remove the burr from where firing pin hit primer. The extractor is very difficult to install once taken apart. It’s not typical. I wasn’t aware federal made nato gold medal match?
 
You can’t push on the back of the bolt carrier?
I tried. The extractor doesn’t move that easy lol. I can probably use some socket ratchet extensions and push it hard enough. But by hand? No way lol
 
No primer or I take a razor blade and remove the burr from where firing pin hit primer. The extractor is very difficult to install once taken apart. It’s not typical. I wasn’t aware federal made nato gold medal match?
Yea that stuff isn’t common but it’s out there.

If you have a nato chamber, your 1.633 gauge will easily chamber as it’s a 308 win sized gauge. I suspect your chamber is 1.635-1.638 (most I’ve seen are in that range). When I measure my 1.635” Forster headspace gauge in the Hornady tool/D400 bushing I get 1.6265”
61A85ABA-BC43-4C51-AB3A-2DD2B8F6D577.jpeg

I’d start there and see if it chambers with the upper off the lower. If so, I’d actually back the die off in .001” increments and resize, measure then attempt to chamber until you get a piece of brass that won’t chamber. Whichever measurement that corresponds to, is essentially your no-go length. Thus your chamber is likely .001-.002” shorter.

Hope that makes sense; if not let me know and I’ll try to frame it differently
 
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Yea that stuff isn’t common but it’s out there.

If you have a nato chamber, your 1.633 gauge will easily chamber as it’s a 308 win sized gauge. I suspect you your chamber is 1.635-1.638 (most I’ve seen are in that range). When I measure my 1.635” Forster headspace gauge in the Hornady tool/D400 bushing I get 1.6265”
View attachment 7672820
I’d start there and see if it chambers with the upper off the lower. If so, I’d actually back the die off in .001” increments and resize, measure then attempt to chamber until you get a piece of brass that won’t chamber. Whichever measurement that corresponds to, is essentially your own-go length. Thus your chamber is likely .001-.002” shorter.

Hope that makes sense; if not let me know and I’ll try to frame it differently
No that makes perfect sense. Question though. If 308 is used, and Using hornady comparator and after fired it measures 1.632”; do I just full length size using the die set at 1.635 and I’m still good to go as long as brass is cut and it fits chamber? Or do I need to drop the 1.632 down to around 1.628”-1.629”
 
No that makes perfect sense. Question though. If 308 is used, and Using hornady comparator and after fired it measures 1.632”; do I just full length size using the die set at 1.635 and I’m still good to go as long as brass is cut and it fits chamber? Or do I need to drop the 1.632 down to around 1.628”-1.629”
Doesn’t matter if 308 or 762 is used, different brass will “over stretch” to different extents with ARs as 918v stated. You have to find the proper measurement via trial and error for the very brass you plan on reloading than size all that brass (virgin or fired) to that length. Once you have the die set up, you should be good to go. Pick one type of 1x fired brass to start.

Once set up, Mark your die, lock ring and press with index marks so you know exactly how it needs to go back. Use that die exclusively for your AR10 and other dies for any other 308/762 rifles you have. The various hash marks on the lock ring represent .002” movement in the shoulder bump.
5F7D15D5-14E2-4DBD-AD1A-0D0DF75FF4F4.jpeg