Gunsmithing Semi-auto .308 checking headspace methods?

[kumaichi]

I'm just getting around to finishing up my SnipersHide JD Machine semi-auto .308 and I'm having some issues with checking the headspace. I bought the Mega Arms .308 18" barrel for this build and when I check the headspace like John McQuay shows in his Maten build (disassemble the bolt, put the GO gauge in the chamber, put the bolt in, and it should spin) the bolt doesn't spin. I contacted Mega Arms and one of the guys over there said I'm doing it wrong. He said that I should assemble the rifle, put the GO gauge in the chamber, pull the bolt back half way, let it go and let the buffer spring seat the bolt, then make sure that the bolt closes all the way.

I really don't want to blow myself up, could someone please advise me as to the best/correct method?

Thanks in advance,

Craig

 

[Delta4-3]

Always check with the bolt and barrel off the weapon if you can, as it illiminates other factors.

 

[smschulz]

The Mega guy is correct.

 

[HawkeyeEngineer]

When you say "disassemble the bolt" do you mean just pull the bolt from the carrier or are you taking the ejector out too? The ejector needs to be out if you are checking the headspace with the bolt out of the upper half. The easiest way to check it that gives good results and won't damage your gages would be to have your upper half completely built, place the muzzle on your work bench and pull the bolt back far enough to insert the gage. Make sure the charging handle is latched shut and then with thumb pressure push the back of the carrier down. If you can't close it with moderate thumb pressure then it won't go.

Pulling it partway back and letting it close may not hurt it, but be careful with your gages. Even though they are (or should be) hardened steel, I've seen a lot of gages damaged from rough handling which will actually throw off your results.

Another thing to consider is what spec your gages are. Headspace range for 308 Winchester is 1.630 to 1.640. Not all manufacturers use the entire headspace range, and not all "GO" gages are actually at 1.630. So you may have a combination of a higher reading gage being used as a "GO" gage on a barrel/bolt combo that could be within SAAMI specs but beneath your "GO" gage.

Hope that helps.

 

[kumaichi]

I took the bolt completely apart (removing the extractor and the ejector) so there would be no resistance when I checked if the GO gauge was really a go. I'm using the Forster .308 family of GO/No GO gauges from Sinclair. When I spoke to the guy at Mega, he said they test them by using some kind of T-handle to put enough force on the bolt to get a "GO condition". I tried pushing pretty hard on the bolt to try and see if it would work but it didn't. I'm just wondering how much pressure it takes to actually test the GO gauge.

 

[HawkeyeEngineer]

Hmm, if I were you I'd try the method I described above using only the upper half. If your gages are good and if you can't push it in with moderate to heavy thumb pressure (without leaning on it!) then I'd say you do not have a "GO" condition and your headspace is too tight. Do you have access to any other gages to try a different set?

 

[kumaichi]

I don't currently have another set of gauges, would you recommend a set, maybe the PTG gauges? I don't have any issues buying new gauges. I will try the method you described tonight when I get home and see if thumb pressure will work. If I can't get it to work, maybe I'll take some video so I can show you what I tried. Thanks for the help on this .

 

[Delta4-3]

The thing about leaving the upper in tact is it's harder to see if there is something up with the foreword side of your lugs causing the issue or if there is a chip left in the chamber, (I had that happen) or a few other scenarios. Its not that anyone else is wrong, it's the whole cat skinning deal...

 

[wilecoyote]

find some unfired,new out of the box .308 Federal Match, take away from them powder,bullet and primer, and try again_ this way, even if you're doin' something wrong, you won't ruin your chamber or your gauge_ would be smart measure the shoulders of the Fed.Match BEFORE and AFTER any try, with or without any force application ,also_

 

[kumaichi]

Since I've been home, this is what I've done. I put the rifle together and put the GO gage in the chamber, brought the bolt back half way and let it go, this did not work so I tried it with the full spring pressure of the buffer spring and it closed successfully on the GO gage. I repeated the procedure with the NOGO gage and the bolt would not close. Before I used the buffer spring, I tried using just my thumbs on the upper to close the BCG and there was no way I could close the bolt. I then put a new Lapua brass case in the chamber and sent the bolt home from the open position. The bolt successfully closed as well.

I'm assuming then, that the barrel has enough headspace in it, would that be a true statement since the bolt closed with the GO gage installed, even though it took the full pressure of the buffer spring to get it to work?

 

[wilecoyote]

I can't fully answer because I haven't enough experience w.rotating etc., coming I from Fal only, where releasing the bolt at full speed on any gauge is a big no-no, and closing it with thumb and index only gentle pinch is ok, on a GO_ I've mentioned Federal Match because, following some reading, generally they are the closest to SAAMI GO specs_(about Lapua ,I can't positively state that) Anyway,sending the bolt home from open position has some meaning ONLY if you have MEASURED the brass shoulder BEFORE that, and re-measured it again AFTER, because the force of the launched bolt can crush the brass,compressing the shoulder, and closing seemingly well, even if the rifle has a wrongly short headspace_said that, the measures obtained, comparated to the measures that you can verify on your "official" Go gauge, can enlighten you a bit_ hope can help

 

[Turd in the Pool]

Sounds like you are GTG. As Wile says, I wouldn't have let her slam home on a gauge. Don't think you damaged anything but it would be a good idea to pull the firing pin and chamber test a sample of your assortment of ammo before heading to the range.

Sucks when they lock up out of battery.

 

[wilecoyote]

let us know if you are a handloader: if your answer is yes, I think you can have some other option before searching troubles_

 

[kumaichi]

I do handload my own ammo . I will try to find some Federal cases and will measure like you said to see if I'm crushing the case, thanks for the suggestion. This whole thing does honestly make me feel uneasy, measuring headspace on a bolt gun is so much easier, lol.

 

[wilecoyote]

ok: now find some cases,for now the brand don't matter, if it's the same brand, full resize them at different shoulder/headspace decreasing values, and see which among these cases will allow you to properly have the bolt closed without forcing_ the key factor is to have a caliper and the Sinclair inserts to measure the best fitting shoulder height, to allow you to write it on paper, keeping it and obtain it again, adjusting accordingly your die_
I say that because the headspace isn't some absolute number given from the Gods, but a conventional measure, therefore if now your rifle is headspaced slightly too much short regarding a "conventional"value, it will suffice reducing consequently the shoulder height of your handloads since from the beginning_
That will spare you to rework your rifle, if it's really only a headspace issue, but it will be mandatory slightlly massaging all your cases even from new, to spare you the experience of some out of battery ignition_
could also be a good thing to avoid sensitive Federal primers, using something as the harder CCI's, to keep your handloads on the safest side_
If my poor English has contributed to take utterly away some clarity from my answer,please, let me know it here or PM/e-mail without problems_
stay safe_

 

[HawkeyeEngineer]

Sounds like you might be on the ragged edge, but doesn't necessarily mean you are unsafe. If you ever get the chance to try another gage set it would probably be good to do so, even if it's the same brand. Even though they are gages there are still tolerances in them. I've seen headspace gages, brand new, vary up to .0007 from the printed dimension (this was confirmed by three sources) even though they were supposed to be held within .0002 of the nominal dimension.

With proper headspace gage design, the tolerances should always make it so there is a chance that they could reject a good barrel but never accept a bad barrel. This is due to safety reasons. So even though MIN headspace for a 308 WIN is 1.630, your specific gage may have a dimension of 1.6302, 1.6305, 1.6307, etc. and it may be rejecting your barrel even though your actual headspace may be right at 1.630 or even 1.6301. Does that make any sense?

Like I said, you may just be on the ragged edge. If your bolt and/or barrel extension is phosphated you can expect the phosphate to wear from the bearing surfaces within the first 100 rounds gaining you back a few .0001's of an inch. Either that or if you have a very small burr it may wear off. That might be just enough for your gage to go easily after break in.

To be safe, after you break your barrel in clean the chamber, barrel extension lugs, and bolt lugs and try the thumb pressure method again and/or use another set of gages if you ever get the chance.

 

[kumaichi]

You guys are great! I've got some PTG headspace gages on order, just so I can verify and make sure it's not a gage issue. Who knows, the gage that Mega used could be out of tolerance as well, anything's possible. I will also try Wiley's suggestion too, if I can get brass that will seat with just thumb pressure that would also give me a warm and fuzzy feeling . I'll keep you guys posted and thanks again for a the suggestions, puts me in a much better perspective .

 

[6brshooter]

Do not pull the firing pin out and try cycling rds or gage. Had a brain fart once on a 6.5 cm i built. Fully assembled upper, pulled the ejector and some reason the FP also. Cam pin rotated and locked the carrier in the forward position. Thought i was going to have to cut the receiver apart, Fulton armory heavy wall upper, got lucky and after jiggling the carrier back and forth was able to run a 17 ga wire through the cam pin to get hole lined up. Stuck FP back in and withdrew the carrier and bolt. Basically shitting down one leg for a hour trying to figure out how to get it apart, get it apart, and avoid calling a smith friend to save embarrassment.

 

[Turd in the Pool]

Do not pull the firing pin out and try cycling rds or gage. Had a brain fart once on a 6.5 cm i built. Fully assembled upper, pulled the ejector and some reason the FP also. Cam pin rotated and locked the carrier in the forward position. Thought i was going to have to cut the receiver apart, Fulton armory heavy wall upper, got lucky and after jiggling the carrier back and forth was able to run a 17 ga wire through the cam pin to get hole lined up. Stuck FP back in and withdrew the carrier and bolt. Basically shitting down one leg for a hour trying to figure out how to get it apart, get it apart, and avoid calling a smith friend to save embarrassment.

Good catch! and an issue I never considered. Hope he's read your post.

 

[kumaichi]

Yep, I'm on it, glad he responded when he did, I was going to try that tonight.

 

[HawkeyeEngineer]

Careful using brass as a method to check headspace. I wouldn't do it at all, not that it will hurt anything but it just won't give you any positive results. Check out the drawing here: http://www.saami.org/PubResources/CC_Drawings/Rifle/308 Winchester.pdf

A conforming (to SAAMI specs) piece of 308 brass can have a headspace dimension .004 larger than a minimum chamber. This means it allows for .004 of "crush" to occur on a MAX cartridge inside of a MIN chamber. So unless you know what the headspace dimension is on that specific piece of brass is it won't do you any good. I'm assuming you don't know that and don't have a way to measure otherwise you'd be able to measure the gage itself. Typically headspace gages can be measured on an optical comparator or with a certified ring gage and height stand.

Let us know what your PTG set shows you. I'm interested to see if another gage gets you a different answer.

 

[6brshooter]

Yeah it sucked, i think i started with a piece of 030 or 035 mig wire. Was finally able to get a FP in. I don't know much, but i do learn from my mistakes and pass that info around.lol

 

[wilecoyote]

about some words above,mine included,please: I'm agreein' that brass isn't a "method to check headspace"_ I need also state again that , beside the proper gauges and their proper use, any work regarding this topic involve having and properly using the caliper and the so now called " Sinclair Comparator/Bump Gage Inserts", or similar means of checking the related measures_ without these items and their proper careful use, the work can't be correctly accomplished,of course_ I hope i'm having been cleary understanded_