Dixie Gunsmithing
Moderator Emeritus
A little late on this, but I remembered it. Bob Dunlap and Headspace gauges.
https://www.youtube.com/watch?v=D-3byRKwO3M
https://www.youtube.com/watch?v=D-3byRKwO3M
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In The Classroom: Headspace Part 2
- Thread starter Dixie Gunsmithing
- Start date
I am not clear on the masking tape. If he is using it just to show that ammo can vary, fine, but if he is using a factory round and masking tape to check headspace in the rifle, that is not a good method simply because ammo is NOT uniform.
We need to use better terminology on headspace and the use of gauges; here is what I would say:
A GO gauge ensures that the longest cartridge that is within specs for the cartridge will chamber and fire. (The gauge should be accepted.)
A NO-GO gauge ensures that the shortest cartridge that is within specs for the cartridge will not stretch far enough in firing to cause head separation and hang up the gun. (The gauge should not be accepted.)
GO and NO-GO gauges are used in factories and by gunsmiths building, rebarrelling or rechambering guns.
A FIELD gauge detects a condition in which the gun is at or near the point at which the shortest cartridge that is within specs MAY have head separation. (The gauge should not be accepted; if it is, the rifle should be repaired. FIELD gauges are used outside the factory - "in the field" - primarily to check out used rifles.)
Jim
We need to use better terminology on headspace and the use of gauges; here is what I would say:
A GO gauge ensures that the longest cartridge that is within specs for the cartridge will chamber and fire. (The gauge should be accepted.)
A NO-GO gauge ensures that the shortest cartridge that is within specs for the cartridge will not stretch far enough in firing to cause head separation and hang up the gun. (The gauge should not be accepted.)
GO and NO-GO gauges are used in factories and by gunsmiths building, rebarrelling or rechambering guns.
A FIELD gauge detects a condition in which the gun is at or near the point at which the shortest cartridge that is within specs MAY have head separation. (The gauge should not be accepted; if it is, the rifle should be repaired. FIELD gauges are used outside the factory - "in the field" - primarily to check out used rifles.)
Jim
Dixie Gunsmithing
Moderator Emeritus
Jim,
He was using the tape to show how reloads could be short, and since the gun closed on a go gauge, but not a no-go, it still had .010" headspace using that round, making it unsafe.
To me, that measurement with the tape doesn't matter, as if the bolt will close on a go, and not on a no-go, then for all intents and purposes, it is safe and to SAMMI spec. If a cartridge were to fail, over being short, then that would put the liability on the cartridge manufacturer, or the reloader, and not the gun, nor the Gunsmith.
He was using the tape to show how reloads could be short, and since the gun closed on a go gauge, but not a no-go, it still had .010" headspace using that round, making it unsafe.
To me, that measurement with the tape doesn't matter, as if the bolt will close on a go, and not on a no-go, then for all intents and purposes, it is safe and to SAMMI spec. If a cartridge were to fail, over being short, then that would put the liability on the cartridge manufacturer, or the reloader, and not the gun, nor the Gunsmith.
Reference for my comments:
http://www.saami.org/PubResources/CC_Drawings/Rifle/30-06 Springfield.pdf
Dixie G, I think that's .010" head clearance; the space between the closed bolt/breech face and case head when the round is fired according to SAAMI. Chamber headspace is measured from the closed breech/bolt face to the chamber point that stops the case from going further into it.
From what I see in SAAMI specs for the .30-06, there can be about .013" head clearance with a minimum spec cartridge (2.0457") in a maximum spec chamber (2.0587"). Is that unsafe?
James K, a SAAMI max spec .30-06 case with 2.0526" headspace will not chamber in a SAAMI minimum spec chamber headspace of 2.0487". The case is about .004" too long in that dimension; the bolt won't properly close on it.
http://www.saami.org/PubResources/CC_Drawings/Rifle/30-06 Springfield.pdf
Dixie G, I think that's .010" head clearance; the space between the closed bolt/breech face and case head when the round is fired according to SAAMI. Chamber headspace is measured from the closed breech/bolt face to the chamber point that stops the case from going further into it.
From what I see in SAAMI specs for the .30-06, there can be about .013" head clearance with a minimum spec cartridge (2.0457") in a maximum spec chamber (2.0587"). Is that unsafe?
James K, a SAAMI max spec .30-06 case with 2.0526" headspace will not chamber in a SAAMI minimum spec chamber headspace of 2.0487". The case is about .004" too long in that dimension; the bolt won't properly close on it.
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Gunplummer
New member
MIL specs used to call their "Chamber point" off the top of the shoulder (30-06, .308) and not where the gauge contacts the shoulder angle, as SAAMI does. Add that to the mix. I have used re-sized brass to headspace many a barrel change with no problem. Others have their methods that work. It certainly seems to me that the internet has opened up a world of headspace problems that do not exist.
Dixie Gunsmithing
Moderator Emeritus
Bart, I doubt it is unsafe, but Bob's videos make it a point to discuss Gunsmithing from a liability standpoint, and I was mentioning what he said in the video. He closed the bolt, which he said was rubbing on the tape, that measured 0.010", and said one shouldn't shoot it. However, if it was a new reload (had been reloaded for the first time), or new ammo, I doubt it would rupture. Bob is forced to comment on safety items that might not happen in the real world, over the liability of the videos. I think what he was trying to get across, was that short ammo, in a gun with good headspace, would act the same as spec ammo that was in a gun with too much headspace.
Gunplummer, check MIL SPECS for the 7.62 NATO chamber headspace and gauges; they reference the same .400 diameter reference about mid point on the shoulder as SAAMI does. Their gauges use the same dimension reference.
From MIL-STD-637 Rev. A for the M14:
"3.15.1 Headspace. The headspace shall be a maximum of 1.640 inches and a minimum of 1.630 inches. Variance from these values shall require Government approval."
However, I do think the original commercial headspace reference on the .30-06 was the shoulder point at 1.9399" length at .4425" diameter. I think I've seen old GO headspace gauges marked 1.9400".
From MIL-STD-637 Rev. A for the M14:
"3.15.1 Headspace. The headspace shall be a maximum of 1.640 inches and a minimum of 1.630 inches. Variance from these values shall require Government approval."
However, I do think the original commercial headspace reference on the .30-06 was the shoulder point at 1.9399" length at .4425" diameter. I think I've seen old GO headspace gauges marked 1.9400".
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"...FIELD gauges are used outside the factory..." They have nothing whatever to do with 'The Field' or a factory. They are test gauges that check manufacturing tolerances only. The Field is the min to max tolerance. This is the Field. "maximum of 1.640 inches and a minimum of 1.630 inches"
Cases do not have headspace.
"...a factory round and masking tape..." That's even worse. That tests nothing at all.
Cases do not have headspace.
"...a factory round and masking tape..." That's even worse. That tests nothing at all.
Dixie G, I think you're right about Bob's headspace comments.
Too many reloaders get into trouble when they size cases as instructed by the manual supplied with the sizing die. Especially if their rifle's chamber is at the longest end of headspace specs. I sure did with my first set of dies used for a rebarreled .308.
Too many reloaders get into trouble when they size cases as instructed by the manual supplied with the sizing die. Especially if their rifle's chamber is at the longest end of headspace specs. I sure did with my first set of dies used for a rebarreled .308.