Hello all....
I've been doing alot of re-barrelling and bolt replacements throughout my M1A/M14S collection and have noticed some things regarding headspace that I thought were both interesting and worth sharing. I don't yet have answers to all the questions I've generated for myself through the exercises however I have some tools on the way that should answer questions and will post updates as info/time permits. I'll also do some speculating as to what I think some of the observations mean, but please do yourself a favor and dissociate the speculation from the observations. For most readers the observations should be the main (if not the ONLY) value of the thread. Readers who find them interesting should interpret them for themselves, coming to their own conclusions. Anyway....I'll get off my soap box now and get going:
Interesting Observation #1: My .308 match grade headspace gauges don't agree with my RCPS precision micrometer:
The first photograph shows the 1.630 go gauge that is being measured inside the precision mic, resulting measurement of 1.557 (negative 3 thou) illustrated in the second photograph. So which one is right? My headspace gauge kit has 9 gauges in it, starting at 1.630 "go" and running up in 1 thou increments to 1.638 "field". Measuring each gauge in the set with the RCBS precision mic shows that they are indeed 1 thou apart from each other, however it still makes me wonder (however unlikely) if the gauges were all made sequentially on a machine that was not calibrated properly. Personally I think what's more likely is that the RCBS gauge is wrong. Anyway....a second set of headspace gauges from a different manufacturer is on the way so I should shortly have more info to report. Also, next time I go down to wholesale sports I'll bring the headspace gauge, get the counter boys to bring out 5 or 6 gauges, and meaure them all to get some idea of variability from one RCBS precision mic to another.
Interesting observation #2: In two chinese M14s's that I've re-barreled with american made barrels and then fitted with TRW bolts to headspace right on at 1.630 (as measured with the above gauge in bolts with no guts), on fired brass from these guns the same RCBS precision mic reads an average of 1.632!?!?! How can the fired brass headspace be 5 thou longer than the headspace as measured by the headspace gauge? (2 thou on the gauge + 3 thou differential between the headspace gauge and the headspace gauge in the RCBS precision mic)
Now for some speculation; (NOT PART OF MY OBSERVATIONS!) here's what I think is happening: these particular barrels started out life as 22 inch take-offs from brand-spanking-new springfield armory M1As that were then cut down to 18.6 inches and re-threaded. In subsequent testing they would not quite cycle with SA surplus ammo, so their gas ports were ever so slightly enlarged. The gas port enlargment effected reliable functioning of these guns, however I suspect it slightly accellerates the action of the piston on the operating rod. When the gun is fired, the brass expands to chamber dimensions under the ~50,000psi. In a normal M1A with a factory length barrel and a standard sized gas port, this pressure bleeds off sufficiently (just as per John Garand's design) to allow the brass to relax somewhat (shrinking just a wee bit as it does) before the op rod has undergone enough rearward travel to have started rearward movement of the bolt. In the case of a cutdown barrel with a slightly larger gas port (ie: NOT per John Garand's design), gas system pressure builds more quickly and acts on the piston/op rod/bolt faster...fast enough that the brass is being stretched from simultaneously being pulled rearward and held via burning propellant pressure agains the chamber walls. In these played with guns the gas system is balanced well enough to still work, if the above speculation is true then the system's also pretty hard on brass; hard enough that case-head separation could be an issue earlier than one might expect.
Interesting Observation #3: In the same guns mentionned above, swapping out the TRW bolts for the original (serial number matched) chicom bolts and then trying to measure headspace with the headspace gauges (bolts disassembled of course!), these guns both SWALLOW the 1.638 field reject gauge...the largest one in the kit! I have not fired either of these guns with the chinese bolts in them, so I don't have a measurement from the RCBS precision mic, however I have commissioned construction of a custom set of "extra long" headspace gauges (from 1.639 through 1.646) that should arrive shortly. In trying to research the differences in acceptable headspace between .308 and 7.62x51, more specifically how long is "too long", no easy answer was out there. Seems like there is some debate as to what field reject for 7.62x51 should be. I'd be curious to read what the general CGN consensus is. Based on what I've read and after talking with some folk for whom I have some significant respect, for now I'm going with 1.645. When the gauges arrive I'll drop them into these two guns with their original bolts as well as a couple of unaltered M14s's still in the grease and then report back with an update to this thread. What do you guys think? How long is too long?
Anyway, that's it for now. Given the number of folk who are making decisions based on measurements from an RCBS precision mic, I figured it was worth posting. For those running chinese bolts I'm also hoping to learn a bit more about "how long is too long". Will report more as tools arrive and I can make additional observations.
Good luck...
Brobee
I've been doing alot of re-barrelling and bolt replacements throughout my M1A/M14S collection and have noticed some things regarding headspace that I thought were both interesting and worth sharing. I don't yet have answers to all the questions I've generated for myself through the exercises however I have some tools on the way that should answer questions and will post updates as info/time permits. I'll also do some speculating as to what I think some of the observations mean, but please do yourself a favor and dissociate the speculation from the observations. For most readers the observations should be the main (if not the ONLY) value of the thread. Readers who find them interesting should interpret them for themselves, coming to their own conclusions. Anyway....I'll get off my soap box now and get going:
Interesting Observation #1: My .308 match grade headspace gauges don't agree with my RCPS precision micrometer:
The first photograph shows the 1.630 go gauge that is being measured inside the precision mic, resulting measurement of 1.557 (negative 3 thou) illustrated in the second photograph. So which one is right? My headspace gauge kit has 9 gauges in it, starting at 1.630 "go" and running up in 1 thou increments to 1.638 "field". Measuring each gauge in the set with the RCBS precision mic shows that they are indeed 1 thou apart from each other, however it still makes me wonder (however unlikely) if the gauges were all made sequentially on a machine that was not calibrated properly. Personally I think what's more likely is that the RCBS gauge is wrong. Anyway....a second set of headspace gauges from a different manufacturer is on the way so I should shortly have more info to report. Also, next time I go down to wholesale sports I'll bring the headspace gauge, get the counter boys to bring out 5 or 6 gauges, and meaure them all to get some idea of variability from one RCBS precision mic to another.
Interesting observation #2: In two chinese M14s's that I've re-barreled with american made barrels and then fitted with TRW bolts to headspace right on at 1.630 (as measured with the above gauge in bolts with no guts), on fired brass from these guns the same RCBS precision mic reads an average of 1.632!?!?! How can the fired brass headspace be 5 thou longer than the headspace as measured by the headspace gauge? (2 thou on the gauge + 3 thou differential between the headspace gauge and the headspace gauge in the RCBS precision mic)
Now for some speculation; (NOT PART OF MY OBSERVATIONS!) here's what I think is happening: these particular barrels started out life as 22 inch take-offs from brand-spanking-new springfield armory M1As that were then cut down to 18.6 inches and re-threaded. In subsequent testing they would not quite cycle with SA surplus ammo, so their gas ports were ever so slightly enlarged. The gas port enlargment effected reliable functioning of these guns, however I suspect it slightly accellerates the action of the piston on the operating rod. When the gun is fired, the brass expands to chamber dimensions under the ~50,000psi. In a normal M1A with a factory length barrel and a standard sized gas port, this pressure bleeds off sufficiently (just as per John Garand's design) to allow the brass to relax somewhat (shrinking just a wee bit as it does) before the op rod has undergone enough rearward travel to have started rearward movement of the bolt. In the case of a cutdown barrel with a slightly larger gas port (ie: NOT per John Garand's design), gas system pressure builds more quickly and acts on the piston/op rod/bolt faster...fast enough that the brass is being stretched from simultaneously being pulled rearward and held via burning propellant pressure agains the chamber walls. In these played with guns the gas system is balanced well enough to still work, if the above speculation is true then the system's also pretty hard on brass; hard enough that case-head separation could be an issue earlier than one might expect.
Interesting Observation #3: In the same guns mentionned above, swapping out the TRW bolts for the original (serial number matched) chicom bolts and then trying to measure headspace with the headspace gauges (bolts disassembled of course!), these guns both SWALLOW the 1.638 field reject gauge...the largest one in the kit! I have not fired either of these guns with the chinese bolts in them, so I don't have a measurement from the RCBS precision mic, however I have commissioned construction of a custom set of "extra long" headspace gauges (from 1.639 through 1.646) that should arrive shortly. In trying to research the differences in acceptable headspace between .308 and 7.62x51, more specifically how long is "too long", no easy answer was out there. Seems like there is some debate as to what field reject for 7.62x51 should be. I'd be curious to read what the general CGN consensus is. Based on what I've read and after talking with some folk for whom I have some significant respect, for now I'm going with 1.645. When the gauges arrive I'll drop them into these two guns with their original bolts as well as a couple of unaltered M14s's still in the grease and then report back with an update to this thread. What do you guys think? How long is too long?
Anyway, that's it for now. Given the number of folk who are making decisions based on measurements from an RCBS precision mic, I figured it was worth posting. For those running chinese bolts I'm also hoping to learn a bit more about "how long is too long". Will report more as tools arrive and I can make additional observations.
Good luck...
Brobee
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