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Polytech M14S Headspace issue

This is a discussion on Polytech M14S Headspace issue within the The M14 forums, part of the M14 M1A Forum category; Hello all, newbie here. I haven't looked around too much yet here, I hope this isn't a USGI forum only, I apologize in advance if ...


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Old February 3rd, 2010, 02:00 PM   #1
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Polytech M14S Headspace issue

Hello all, newbie here.

I haven't looked around too much yet here, I hope this isn't a USGI forum only, I apologize in advance if I offend anyone asking about a chicom rifle here.

I know it has probably been covered many times but I am still unsure of the correct answer so hopefully I have come to the right place.

I recently purchased a Polytech M14s from a Pawn Broker. It looked unused and was a good price so I took the chance. Before firing it I took it to a gunsmith to check the headspace and it failed the field gauge test (using a .308 field gauge). He tagged it as unsafe, said it was toast.

I went online and have found many conflicting reports but many say that most of these will fail the headspace check when using a commercial .308 field gauge and they are often fine when checked with a NATO gauge.

I called the gunsmith back about this and he berated me for questioning him me telling me there is no difference (even though many online sources say differently) He told me to take a hammer and smash my computer because he was right and it was giving me the wrong info!!

One might have thought he was saying this to get my rebuild business but he didn't seem interested (and quite frankly not sure he knew what he was talking about because he said the only way to fix the problem is to get a new barrel, he literally spent 2 minutes with the gun so I am not sure how he came up with what the proper fix was for it)


I then called a few other gunsmiths, one concurred with the first guy, another said it is not going to explode on me, just use it.


So what is the truth? Do I need to fix this? Can you normally fix this with a bolt replacement or do you need to go the whole nine yards?

I realize a definitive answer is not possible without seeing the gun but if you can help separate the wheat from the chafe I would greatly appreciate it.


Last edited by mellc1; February 4th, 2010 at 09:37 AM.
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Old February 3rd, 2010, 02:45 PM   #2
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Ploy M-14's are head spaced for 7.62 X 51 NATO checking the head space with a 308 gauge it could fail and still be safe to shoot 7.62 X51NATO.
Get a NATO 7.62X 51 gauge and check it again. Ploy M-14 are know en for long head space and soft bolts.Find a gunsmith that works on m14

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Old February 3rd, 2010, 02:49 PM   #3
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There is a difference between 7.62 Nato and .308 Winchester headspace gauges. Don't take my word for it though. Read Gus Fisher's exposition on headspace in his section of the forum. The Kuhnhausen shop manual would also beg to differ. I would recommend asking a smith that specializes in M14 work to check it for an exact measurement to see what you are dealing with. The difference will be the ammo you shoot and the type of cases you can use and how you set up your sizing die if you reload. You may be able to swap bolts with a GI bolt to get it back into spec although that should be done by a M14 smith who knows what he's doing.

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Old February 3rd, 2010, 03:31 PM   #4
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That first gunsmith was so smug, arrogant and obviously ignorant, I wonder how many people he has steered wrong? He actually did tell me what I needed to do was smash my computer because the info I was getting online was wrong!!

So any recommendations on a Nato field gauge?

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Old February 3rd, 2010, 03:39 PM   #5
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You more than likly you only needs to have the bolt replaced with a USGI one. The locking lugs are not machined right on the bolt causing long headspace. The bolt for a polytech is a fitted part and must be done correctly. Fulton Armory makes a good book about polytech/M14 rifles.
It shows how to fit a USGI bolt in a poly. You can reuse the poly barrel if it headspace correctly.

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Old February 3rd, 2010, 06:30 PM   #6
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Quote:
Originally Posted by bigblock View Post
You more than likly you only needs to have the bolt replaced with a USGI one. The locking lugs are not machined right on the bolt causing long headspace. The bolt for a polytech is a fitted part and must be done correctly. Fulton Armory makes a good book about polytech/M14 rifles.
It shows how to fit a USGI bolt in a poly. You can reuse the poly barrel if it headspace correctly.

+1 Get the Fulton armory book. It has instructions for this.

I have heard reports of side stamped polys able to accept a GI bolt with no modifications.

I was not so lucky. I had to fit my bolt and change out the barrel. It can be done by yourself if you have decent mechanical skills and are willing to invest in some tools.

Most likely you are good to go. Just use military 7.62nato ammo and check headspace regularly

Thanks from echokilo60
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Old February 3rd, 2010, 06:35 PM   #7
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Almost forgot. The gunsmith you took your rifle too is not well versed in military arms. Most are not and have no idea on them.

I talked to a local smith near my area and he went on and on about how "hard" it is to headspace barrels on the m14 because they are timed barrels. He then flat out refused to even look at my rifle.

If you want to have your rifle rebuilt by a professional send it to a M14 smith.

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Old February 3rd, 2010, 07:10 PM   #8
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Info on the difference between the 7.62 and .308 chambers.

http://docs.google.com/Doc?docid=0AS...c2M3c2Zn&hl=en

SAAMI Spec sheet for the .308 cartridge and chamber

http://docs.google.com/fileview?id=0...ZWE1MTU3&hl=en

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Old February 4th, 2010, 07:15 AM   #9
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Check Numrich Arms on line as they usually have the 7.62 NATO field gage. 2 minutes does not seem to be enough time to check headspace as u have to dissassemble the bolt.

Be aware of the difference in pressure betwen 7.62 NATO rounds and commercial 308 and use the commercial 308 sparingly as the SAAMI standard for commercial is significantly greater than the NATO standards.

Don

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Old February 4th, 2010, 07:52 AM   #10
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Old February 4th, 2010, 08:17 AM   #11
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Question Unsafe?

Your Poly might have a soft bolt. If it does the headspace is only going to increase as you shoot it. Mine did. Get your bolt checked at a machine shop that has a Rockwell tester to see if it's too soft before you go any further.

Headspace gauges vary in measurement between manufacturers, so the only accurate way to tell what you've got is to have the measurement itself.

Most specs say that a 7.62 NATO field gauge measurement is 1.6445". That's pretty loose, and firing factory or handloaded ammo (either SAAMI or NATO) might or might not cause a blow-up but it very more than likely will result in many case head seperations.

Ideally, if you're going to have the bolt replaced and possibly rebarrel you should shoot for a minimum headspace measurement of 1.631" to 1.633" which will allow you to safely fire both NATO and SAAMI spec. ammo. I'm not saying that a slightly longer measurement is unsafe, note the word "ideally'.

Gunsmiths have to be very concerned with liability issues in this day and age so don't be too critical of your 'smith, as arrogant as he may seem to be. However, if I were you I'd find someone who specializes in M14's to work on this rifle.

Thanks from echokilo60
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Old February 4th, 2010, 09:45 AM   #12
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Thanks guys, for clearing things up.

From some other sources I have found, its not that the bolt is "soft", its this;

"The other major problem is that the bolt lug angle is sometimes off a bit causing mating discrepancies in the receiver.
I think this is the actual cause of the increasing headspace problems.
The bolt steel on 2 of ours has been tested and is Rockwell 46 so My opinion is that the bolts are NOT soft"

I had found a weird history regarding the start of the claims of soft bolts , why it is a myth (not just by companies trying to get you to spend money doing a rebuild), according to many. I can't even remember any of the context, I will post it when I find it again

Thanks from macku72

Last edited by mellc1; February 4th, 2010 at 10:05 AM.
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Old February 4th, 2010, 10:06 AM   #13
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Quote:
Originally Posted by Seabreeze133 View Post
Check Numrich Arms on line as they usually have the 7.62 NATO field gage. 2 minutes does not seem to be enough time to check headspace as u have to dissassemble the bolt.

Be aware of the difference in pressure betwen 7.62 NATO rounds and commercial 308 and use the commercial 308 sparingly as the SAAMI standard for commercial is significantly greater than the NATO standards.

Don
To make this a bit clearer...
You can get a copy of the specs on the 7.62 cartridge and chamber at;

http://docs.google.com/leaf?id=0By0j...Y2Q5MmQ0&hl=en

My previous post will give you the specs on the .308.

The specs on both the 7.62 and the .308 can also be found in the book

The U.S. .30 Caliber Gas Operated Service Rifles
A Shop Manual
Volumes I&II
by Jerry Kuhnhausen

on pages 43 & 44.

Note that the spec for the diameter of the head portion of the cartridge on the
7.62 is
0.4633" - 0.4693"

and for the .308 it is
0.4623" - 0.4703"

thats 0.006" and 0.008" variance respectively. So the .308 cartridge can actually be manufactured in accordance with SAAMI and it would have a wider allowed variance for the cartridge head dimension than the 7.62 and whats more the .308 cartridge's head can be as much as a thousandths of an inch smaller than the 7.62 or as much as a thousandths of an inch larger than the 7.62's head dimension. So if you have a rifle that was chambered for a tight 7.62 cartridge spec and you use a commercial cartridge that was made to conform to the top end of the SAAMI spec then you could experience chambering issues because the commercial cartridge's head diameter would be too large for the chamber dimension of the rifle. I believe that the head space dimension is like this also, the SAAMI spec has a greater variance than the 7.62 spec by 0.001".

Thanks from Mainzer
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Old February 4th, 2010, 11:35 AM   #14
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Mellc1, Could you recite the source of you Quotations of the aforementioned descriptions. Just Curious.

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Old February 4th, 2010, 11:47 AM   #15
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Quote:
Originally Posted by Kid Sopris View Post
Mellc1, Could you recite the source of you Quotations of the aforementioned descriptions. Just Curious.
Looks like a thread from the errornet via the Sniper Country forum

http://www.snipercountry.com/HotTips/HighEndRifles.htm

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