M1A/M14S/M305 Headspace
- Thread starter Brobee
- Start date
CF Headspace specs-FNC1 and C2
I never throw too much out and today came across page 1 of Canadian Army EME Manual Weapons C820 30 Mar 62( re-numbered to CFTO C-71-113-000/MA-000) which is the data summary on the FN rifle.Headspace specs are as follows(in relation to the .400 datum line on the gauge):
Field and Base-1.6315 minimum
Field-1.640 maximum
Base-when fitting new barrel or shoulder, locking-1.6378 maximum.
Note that there is no mention of any other gauges.This and the US Army TM9-1005-223-35 dated July 1968 ,which describes a field reject gauge of 1.6455,are the only official documents which I have seen giving headspace specs on the 7.62 NATO chamber.FWIW.
I never throw too much out and today came across page 1 of Canadian Army EME Manual Weapons C820 30 Mar 62( re-numbered to CFTO C-71-113-000/MA-000) which is the data summary on the FN rifle.Headspace specs are as follows(in relation to the .400 datum line on the gauge):
Field and Base-1.6315 minimum
Field-1.640 maximum
Base-when fitting new barrel or shoulder, locking-1.6378 maximum.
Note that there is no mention of any other gauges.This and the US Army TM9-1005-223-35 dated July 1968 ,which describes a field reject gauge of 1.6455,are the only official documents which I have seen giving headspace specs on the 7.62 NATO chamber.FWIW.
purple: Good find! Interesting to note the two different specs for 7.62 NATO FIELD REJECT....makes it easier to understand why there are no commercially available 7.62 NATO headspace gauges.
Leg:
Problem #1: What is the headspace of the chinese guns? I have gauges on the way to answer this question.
Problem #2: What are the headspace specs for 7.62x51 NATO?
I think that what you are referring to in the "sloppy, yet acceptable" department is the NO-GO gauge. The FIELD REJECT is supposed to be the absolute maximum safe chamber length. Seems kinda counter-intuitive to me...but that's how all my other headspace gauges are rigged up (GO is the shortest, NO-GO is in the middle, and FIELD is the longest).
Brobee
Leg:
Problem #1: What is the headspace of the chinese guns? I have gauges on the way to answer this question.
Problem #2: What are the headspace specs for 7.62x51 NATO?
I think that what you are referring to in the "sloppy, yet acceptable" department is the NO-GO gauge. The FIELD REJECT is supposed to be the absolute maximum safe chamber length. Seems kinda counter-intuitive to me...but that's how all my other headspace gauges are rigged up (GO is the shortest, NO-GO is in the middle, and FIELD is the longest).
Brobee
A field gauge is generally thought to be an upper limit, suggesting that attention is needed. If a 7.62 rifle were to swallow a milspec. field gauge it could be considered to be on the loose side.
My 305 would accept a SAAMI (not NATO) field gauge. I traded for a M-14 bolt, and now it will close on a SAAMI nogo, but not the field. I think that this about right for an autoloading 7.62 rifle.
If a 305 or M-1 is be rebarrelled, a pull reamer driven from the muzzle is the best way to go. Repeatedly removing, reaming and replacing a barrel is inconvenient at best. Ream a bit too deep, and the barrel cannot be set back to correct.
There is a lot of information about handloading for these rifles. Handloading introduces a whole other set of considerations respecting headspace.
My 305 would accept a SAAMI (not NATO) field gauge. I traded for a M-14 bolt, and now it will close on a SAAMI nogo, but not the field. I think that this about right for an autoloading 7.62 rifle.
If a 305 or M-1 is be rebarrelled, a pull reamer driven from the muzzle is the best way to go. Repeatedly removing, reaming and replacing a barrel is inconvenient at best. Ream a bit too deep, and the barrel cannot be set back to correct.
There is a lot of information about handloading for these rifles. Handloading introduces a whole other set of considerations respecting headspace.
UPDATE: Custom Gauges arrived.
2 brand-spanking-new Norinco M14Ss checked out; one at 1.640 and the other at 1.641. I'm happy they're within 7.62 NATO spec.
As with my other gauges (some from other manufacturers), my RCBS Precision Mic measures these new gauges to be 3 thou shorter than their indicated length. I wentdown to wholesale with a headspace gauge to try and compare several Precision Mics against each other, however they were out of stock and had none on order....
Given Lazerus2000's thoughts on tight headspace (now a part of the sticky thread on re-barreling and headspace) and the popularity of tight fittin USGI bolts, I'll try to follow up on this. Right now I'm believing my headspace gauges over the precision mic and will be using 1.632 as the minimum headspace in pretty much all my M1A/M14Ss. As many of my guns are setup right on 1.630, I have some lapping to do.....
Brobee
2 brand-spanking-new Norinco M14Ss checked out; one at 1.640 and the other at 1.641. I'm happy they're within 7.62 NATO spec.
As with my other gauges (some from other manufacturers), my RCBS Precision Mic measures these new gauges to be 3 thou shorter than their indicated length. I wentdown to wholesale with a headspace gauge to try and compare several Precision Mics against each other, however they were out of stock and had none on order....
Given Lazerus2000's thoughts on tight headspace (now a part of the sticky thread on re-barreling and headspace) and the popularity of tight fittin USGI bolts, I'll try to follow up on this. Right now I'm believing my headspace gauges over the precision mic and will be using 1.632 as the minimum headspace in pretty much all my M1A/M14Ss. As many of my guns are setup right on 1.630, I have some lapping to do.....Brobee
Brobee,
lapping is meant to mate the pieces together for perfect bearing, not to adjust headspace. While I have admittedly lapped out .003" or more on occasion, this much change was mostly due to getting the previously uneven mating surfaces to match up. Once you get to full bearing on the left lug, I would advise you to stop there.
The tough/hard outer skin on the bolt and receiver is only a few thou thick. If you lap too far you can get into the soft chewy caramel part at the center.
If I had 1.630" in a TARGET rifle, which is to be used with MATCH ammo and also cleaned regularly, I would be happy with that. If you hand feed [ with a stripped bolt and without the recoil spring ] several different brands of 7.62 NATO ammo, you will probably find that none of them are tight in even a 1.630" chamber. I especially like Hirtenberg ammo in the M-14. Hand feeding the ammunition of choice to test for fit is the ultimate reality check.
PS: If you have several M-14 rifles, swapping the bolts around to find the best pre-lapping headspace before starting with the lapping, will probably show a LOT of variation in the bolts. I've seen up to .010" difference between "NEW" bolts tried in the same chamber.
good luck
LAZ
lapping is meant to mate the pieces together for perfect bearing, not to adjust headspace. While I have admittedly lapped out .003" or more on occasion, this much change was mostly due to getting the previously uneven mating surfaces to match up. Once you get to full bearing on the left lug, I would advise you to stop there.
The tough/hard outer skin on the bolt and receiver is only a few thou thick. If you lap too far you can get into the soft chewy caramel part at the center.
If I had 1.630" in a TARGET rifle, which is to be used with MATCH ammo and also cleaned regularly, I would be happy with that. If you hand feed [ with a stripped bolt and without the recoil spring ] several different brands of 7.62 NATO ammo, you will probably find that none of them are tight in even a 1.630" chamber. I especially like Hirtenberg ammo in the M-14. Hand feeding the ammunition of choice to test for fit is the ultimate reality check.
PS: If you have several M-14 rifles, swapping the bolts around to find the best pre-lapping headspace before starting with the lapping, will probably show a LOT of variation in the bolts. I've seen up to .010" difference between "NEW" bolts tried in the same chamber.
good luck
LAZ
7.62 NATO headspace
Good that you found these within specs.The tolerances between "go" and "field" specs on the 7.62 (assuming you accept "GO" to be 1.631 per CFTO and "FIELD" to be 1.6455 per US Army TM) are .0145.This is quite instructive when you compare the same tolerances on other popular rounds;.30-06 is .010,.303 Brit is .010,and .308 Win is .008.Generally tighter is better,so once "GO" is established on a chamber,I would keep the upper limit as close as possible to that measurement.For the 7.62,I think that 1.632 is probably the optimum "GO" dimension,with 1.638 or 1.640 a desirable upper limit-assuming MILSPEC brass is used and reloading is contemplated.You can safely use MILSPEC brass in a 7.62 chamber which qualifies on the 1.6455 "FIELD" gauge.There is considerable temptation for 7.62 shooters to use .308 Win cartridges or brass,and they should be cautioned against doing so in a 7.62 chamber which exceeds the SAAMI "FIELD" measurement of 1.638.
Good that you found these within specs.The tolerances between "go" and "field" specs on the 7.62 (assuming you accept "GO" to be 1.631 per CFTO and "FIELD" to be 1.6455 per US Army TM) are .0145.This is quite instructive when you compare the same tolerances on other popular rounds;.30-06 is .010,.303 Brit is .010,and .308 Win is .008.Generally tighter is better,so once "GO" is established on a chamber,I would keep the upper limit as close as possible to that measurement.For the 7.62,I think that 1.632 is probably the optimum "GO" dimension,with 1.638 or 1.640 a desirable upper limit-assuming MILSPEC brass is used and reloading is contemplated.You can safely use MILSPEC brass in a 7.62 chamber which qualifies on the 1.6455 "FIELD" gauge.There is considerable temptation for 7.62 shooters to use .308 Win cartridges or brass,and they should be cautioned against doing so in a 7.62 chamber which exceeds the SAAMI "FIELD" measurement of 1.638.
Laz: In mixing and matching all my TRW bolts across the various receivers I have, I was "lucky" enough to wind up with 2 that headspace 1.630 and another couple that headspace 1.631. I will hand check them all with SA surplus as per your suggestion. None of my TRW bolt/norinco receiver combinations will result in 1.632.... Do you think that lapping them for the 1 to 2 thou would be bad? On the other hand, I suppose that headspace is going to grow a bit as the lugs wear under operation...these are the guns I shoot the most and I run a fair amount of ammo through them in a year...
Do you think that lapping them for the 1 to 2 thou would be bad? On the other hand, I suppose that headspace is going to grow a bit as the lugs wear under operation...these are the guns I shoot the most and I run a fair amount of ammo through them in a year...
Brobee,
Lap to fit the left lug with full engagement ... that is the most important part of the headspace/bolt fit equation. A few thou either way on headspace won't matter any where near as much as the left lug bearing full. If you have only a tiny bit of left lug bearing, the bolt can wear very quickly [ if hard ] or get peened back [ if soft ]. This can #### the bolt at an angle.
Remember, GI tolerances were designed for MUD, rough ammo, and people who cleaned their rifles every ten years ... whether they needed it or not.
In other words, the worst case scenario. You sound like the type of shooter who would keep your equipment clean, and use good ammo.
PS: In my bolt action Savage 110 C, which had a DCRA 7.62 NATO heavy target barrel fitted to it, I turned the barrel down tight on a .308 Win GO 1.630" gauge. It still had no problem chambering 7.62 NATO Hirtenberg ammo easily, with no marks on the case.
as usual,
YMMV
LAZ 1
[;{)
Lap to fit the left lug with full engagement ... that is the most important part of the headspace/bolt fit equation. A few thou either way on headspace won't matter any where near as much as the left lug bearing full. If you have only a tiny bit of left lug bearing, the bolt can wear very quickly [ if hard ] or get peened back [ if soft ]. This can #### the bolt at an angle.
Remember, GI tolerances were designed for MUD, rough ammo, and people who cleaned their rifles every ten years ... whether they needed it or not.
In other words, the worst case scenario. You sound like the type of shooter who would keep your equipment clean, and use good ammo.
PS: In my bolt action Savage 110 C, which had a DCRA 7.62 NATO heavy target barrel fitted to it, I turned the barrel down tight on a .308 Win GO 1.630" gauge. It still had no problem chambering 7.62 NATO Hirtenberg ammo easily, with no marks on the case.
as usual,
YMMV
LAZ 1
[;{)
Brobee,
a few other thoughts ...
the NEW TRW bolts I've checked were consistently about .003' - .005' longer = tighter than the other brands. This is one of the few places, where the TRW stuff actually WAS better than the others. And this is one of the reasons [ aside from brand snobbery ] that the NEW TRW bolts are so desireable [ and so expensive ].... they actually can tighten up a loose chamber by a few thou.
To increase headspace clearances a bit, you might want to invest in a NEW bolt of a different manufacturer, or else in a USED bolt. Careful with the used M-14 bolts ... I was at a big gun show in the US and saw a box of M-14 bolts on a table for a reasonable price. They LOOKED new [ reparked ] but when I checked them carefully, I didn't buy any of them.
The lug wear pattern [ especially at the left lug ] is the most reliable indicator of wear and proper bolt fit on the M-14 system. Dropping a GI bolt into a Chinese receiver, without lapping the two together, is merely the starting point. Once you lap the lugs in, then is the time to start checking headspace seriously. This would be especially true of USED bolts, as the pre-existing wear has to be mated up to the Chinese receiver cutouts.
Believe it or not, the Chinese receivers I've worked with actually varied in cutout configuration at the left lug, less than the CAST Springfields. The same is true of the Op Rod slot cutouts ... some CAST Sprongfield receivers take hours of work to get a GI Op rod to fit.
The right receiver lug is also important to reliable bolt camming, as some of the Chinese receivers have too hard a corner at the top, where it transitions from horizontal to vertical. The last one i got was actually sharp enough to draw blood. Breaking this corner to a nice radius will smooth out bolt and op rod travel considerably.
Most important thing about the right lug fit, is that the bolt lug goes all the way down into the cut out, without the bearing hitting the receiver. On some Chinese, and especially some CAST Springfields, you need to grind a bit of a half round relief cut there, so the bearing is not the part stiopping the cycle on chambering. Otherwise the bearing will NOT last long.
hope this helps,
and just out of curiousity,
how many M-14 rifles do you have now?
[;{)
LAZ 1
a few other thoughts ...
the NEW TRW bolts I've checked were consistently about .003' - .005' longer = tighter than the other brands. This is one of the few places, where the TRW stuff actually WAS better than the others. And this is one of the reasons [ aside from brand snobbery ] that the NEW TRW bolts are so desireable [ and so expensive ].... they actually can tighten up a loose chamber by a few thou.
To increase headspace clearances a bit, you might want to invest in a NEW bolt of a different manufacturer, or else in a USED bolt. Careful with the used M-14 bolts ... I was at a big gun show in the US and saw a box of M-14 bolts on a table for a reasonable price. They LOOKED new [ reparked ] but when I checked them carefully, I didn't buy any of them.
The lug wear pattern [ especially at the left lug ] is the most reliable indicator of wear and proper bolt fit on the M-14 system. Dropping a GI bolt into a Chinese receiver, without lapping the two together, is merely the starting point. Once you lap the lugs in, then is the time to start checking headspace seriously. This would be especially true of USED bolts, as the pre-existing wear has to be mated up to the Chinese receiver cutouts.
Believe it or not, the Chinese receivers I've worked with actually varied in cutout configuration at the left lug, less than the CAST Springfields. The same is true of the Op Rod slot cutouts ... some CAST Sprongfield receivers take hours of work to get a GI Op rod to fit.
The right receiver lug is also important to reliable bolt camming, as some of the Chinese receivers have too hard a corner at the top, where it transitions from horizontal to vertical. The last one i got was actually sharp enough to draw blood. Breaking this corner to a nice radius will smooth out bolt and op rod travel considerably.
Most important thing about the right lug fit, is that the bolt lug goes all the way down into the cut out, without the bearing hitting the receiver. On some Chinese, and especially some CAST Springfields, you need to grind a bit of a half round relief cut there, so the bearing is not the part stiopping the cycle on chambering. Otherwise the bearing will NOT last long.
hope this helps,
and just out of curiousity,
how many M-14 rifles do you have now?
[;{)
LAZ 1
I just googled a few neat sources for M-14 info.
For a slightly different perspective on the GI bolt to Chinese receiver issues.
http://www.westtexasarmory.com/FAQ.htm
NOTE: this site is American, so he is NOT talking about our new Candaian Chinese imports, which do NOT have the longer barrel shroud.
***********************************
The definitive info source for all things related to M-14 gunsmithing still has to be the Kuhnhausen book:
http://www.sydweedon.com/kuhnhausen.asp
*************************************
want to maintain your M21 sniper rfilfe??
http://www.globalsecurity.org/military/library/policy/army/fm/23-10/Appb.htm
enjoy,
[;{)
LAZ 1
For a slightly different perspective on the GI bolt to Chinese receiver issues.
http://www.westtexasarmory.com/FAQ.htm
NOTE: this site is American, so he is NOT talking about our new Candaian Chinese imports, which do NOT have the longer barrel shroud.
***********************************
The definitive info source for all things related to M-14 gunsmithing still has to be the Kuhnhausen book:
http://www.sydweedon.com/kuhnhausen.asp
*************************************
want to maintain your M21 sniper rfilfe??
http://www.globalsecurity.org/military/library/policy/army/fm/23-10/Appb.htm
enjoy,
[;{)
LAZ 1
UPDATE -> Clymer vs Forster: Clymer GO and NO-GO gauges arrived yesterday. Using the precision mic, they measure within 0.0005 of the Forster gauges. I'm convinced that my RCBS precision mic was not calibrated right and measures things 0.003 shorter than they really are.
UPDATE -> 7.62x51 vs .308 SAAMI: It was a seriously beautiful day yesterday so I played hooky from work to get some range time. Ran approximately 150 rounds through my favorite 18.5 inch barreled gun which is headspaced right at 1.630 with cutdown springfield armory barrel and TRW bolt. Came home and used the precision mic to measure the brass:
Remember that my RCBS precision mic measures a 1.630 GO gauge as 1.627. Keeping this in mind that's 5 thou brass stretch on the SA surplus, 3 thou brass stretch on the winchester whitebox, and 2 thou brass stretch on the handload. I forgot to bring the screwdriver with me to test them all with the spindle valve turned off...d'oh!...found the differences in brass stretch interesting though and I'm wondering if they're any indication of pressure; both in terms of absolute maximum pressure as well as area under the pressure-time curve. If so, I'd speculate that the SA surplus (7.62x51 NATO spec) ammo in the short 1.630 chamber does produce higher pressure than commercially loaded .308 wwb or my varget propelled (slow burning powder...some would argue too slow) handload.
Lazerus2000: Thank you sincerely for reams of high quality info and the sharing of your obviously extensive experience. All your bolt fitting advice is golden and will be carefully considered in tweeking all my guns. Most of my bolts are TRW; I have one HRT which seems to be pretty much the same headspace wise as the TRWs....it fits maybe 1 thou longer. As for how many M1A/M14s/M305s I have...the answer is TOO MANY! I have a couple springfields, more than a half dozen chinese variants, and hopefully soon an LRB (just to have one). Thanks again for your help.
Brobee
UPDATE -> 7.62x51 vs .308 SAAMI: It was a seriously beautiful day yesterday so I played hooky from work to get some range time. Ran approximately 150 rounds through my favorite 18.5 inch barreled gun which is headspaced right at 1.630 with cutdown springfield armory barrel and TRW bolt. Came home and used the precision mic to measure the brass:
- 1.632 - 15 case average for SA surplus.
- 1.630 - 15 case average for commercially available .308 winchester whitebox 147gr FMJ
- 1.629 - 15 case average for my most accurate handload: winchester brass, cci large rifle primer, 42gr varget, 168gr Nosler J4 competition HPBT, 2.800 COL
Remember that my RCBS precision mic measures a 1.630 GO gauge as 1.627. Keeping this in mind that's 5 thou brass stretch on the SA surplus, 3 thou brass stretch on the winchester whitebox, and 2 thou brass stretch on the handload. I forgot to bring the screwdriver with me to test them all with the spindle valve turned off...d'oh!...found the differences in brass stretch interesting though and I'm wondering if they're any indication of pressure; both in terms of absolute maximum pressure as well as area under the pressure-time curve. If so, I'd speculate that the SA surplus (7.62x51 NATO spec) ammo in the short 1.630 chamber does produce higher pressure than commercially loaded .308 wwb or my varget propelled (slow burning powder...some would argue too slow) handload.
Lazerus2000: Thank you sincerely for reams of high quality info and the sharing of your obviously extensive experience. All your bolt fitting advice is golden and will be carefully considered in tweeking all my guns. Most of my bolts are TRW; I have one HRT which seems to be pretty much the same headspace wise as the TRWs....it fits maybe 1 thou longer. As for how many M1A/M14s/M305s I have...the answer is TOO MANY! I have a couple springfields, more than a half dozen chinese variants, and hopefully soon an LRB (just to have one). Thanks again for your help.
Brobee
Brobee said:
Here is a link to my Chrono of .308 and 7.62x51:
http://www.canadiangunnutz.com/forum/showthread.php?t=144098&page=2
hfp75 said:
Update:
Custom headspace gauges arrived, as well as Jerry K's AWESOME shop manual on the M1 and M14 type rifles. He refers to 7.62 NATO FIELD as 1.6455 and given the depth and bredth of info throughout his manual, this is the number I'm going to go with.
Out of 8 norinco's (all bought within the last 4 years) I checked with their stripped factory bolts, most were in and around the 1.640 - 1.642 range. Two of them however were longer than the longest headspace gauge I have (1.645)...too long for my comfort! Replacing the factory bolts on these two with TRW bolts, they came in at 1.632 and 1.633...
Custom headspace gauges arrived, as well as Jerry K's AWESOME shop manual on the M1 and M14 type rifles. He refers to 7.62 NATO FIELD as 1.6455 and given the depth and bredth of info throughout his manual, this is the number I'm going to go with.
Out of 8 norinco's (all bought within the last 4 years) I checked with their stripped factory bolts, most were in and around the 1.640 - 1.642 range. Two of them however were longer than the longest headspace gauge I have (1.645)...too long for my comfort! Replacing the factory bolts on these two with TRW bolts, they came in at 1.632 and 1.633...
Brobee, thankyou for all the first hand info, you to Lazerus.
Excess headspace does not make rifles unsafe to fire or make them blow up. It does make reloading for them harder and harder on brass. If I was using a M-305 in Afganistan or Iraq, I would want loose headspace, it leaves a little room for some sand.
Tight headspace is better for reloaders and range commandos like me.
Excess headspace does not make rifles unsafe to fire or make them blow up. It does make reloading for them harder and harder on brass. If I was using a M-305 in Afganistan or Iraq, I would want loose headspace, it leaves a little room for some sand.
Tight headspace is better for reloaders and range commandos like me.
Interesting read... A little over my head..
Except the precision mic. and *Go/No-Go Gauge* Stuff.... I use these daily on the *Job*
How could anyone take a $20-$30 Precision gauge seriously?? Is that all that is available?? I would never trust something that cheap. If these measurements are so critical, there must be something better out there??
I'm not into reloading...but I thought things were better quality
A proper Certified Go/No-Go Gauge (four decimal places) runs a good $500.
*note.... Just an example. The RCBS Mic.
1) What is the coating on it?
2)Mics. should not have a coatng.
3)How thick is the coating?
4)How is the thickness of the coating controlled?
5)What happens when it starts to wear? (measurments off)
6)What kind of metal is it? Hardness?
7)What happens when the metal wears? (measurments off)
Except the precision mic. and *Go/No-Go Gauge* Stuff.... I use these daily on the *Job*
How could anyone take a $20-$30 Precision gauge seriously?? Is that all that is available?? I would never trust something that cheap. If these measurements are so critical, there must be something better out there??
I'm not into reloading...but I thought things were better quality
A proper Certified Go/No-Go Gauge (four decimal places) runs a good $500.
*note.... Just an example. The RCBS Mic.
1) What is the coating on it?
2)Mics. should not have a coatng.
3)How thick is the coating?
4)How is the thickness of the coating controlled?
5)What happens when it starts to wear? (measurments off)
6)What kind of metal is it? Hardness?
7)What happens when the metal wears? (measurments off)
The cost is in the certification and documentation and in the fact that it is a one off.
To make one of these chamber guages a little longer or shorter doesn't require much more set up and no more run time than other normal product. In house inspection doesn't charge the same amount as a certification house does.
While the guage may be your 'standard', it isn't any thing more than production for the manufacturer. His 'standards' for inspection do cost more.
**Edited to add**
I am not sure of the liability issue but if the manufacturer's standards are off but certified correct, wouldn't the certification house be responsible for the dead product?
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Too much headspace can result in case-head separation. In a case of severe head separation, it is possible that the seal between the brass and the chamber wall is broken, allowing hot propellent gasses to vent rearwards against (or around) the bolt in ways the bolt was not designed for. Best case scenario....shooter is grumbling about having to find a case extractor to get the rifle up and running again. Worst case, they'd be picking brass and maybe some bolt steel out of their face...
I'd agree that in M14 type rifles, more ka-booms are likely caused by tight headspace (on top of a bunch of other compounding factors), but I don't agree that too much headspace can't also contribute to a possible KB.
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