Check headspace???

[Curlew]

Headspace gauge data starts on page
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Z299.4 - 212 (224)

Thanks — I had missed that section. It does give a blueprint for the gauge, but as far as I can see SAAMI remains essentially silent on the question of what length dimensions to use for “GO” and “NOGO” (never mind “FIELD”). They just have to be somewhere between chamber MIN and MAX.

So e.g. if I wanted to emulate what Sam Colt and Uncle Sam use for the M16 chamber, I could make a pair of gauges per the Z299.4 print, one with F=1.4646 and another with F=1.4706, labeled “Sams GO 223 Rem” and “Sams NOGO 223 Rem” respectively, and that would comply with SAAMI guidance.

My point is basically to echo Todd.K: if the gauge doesn’t have a length marked on it, it’s hard to know what you’ve got. If it’s a military gauge made to a specific drawing, great. But if it’s a commercial gauge that just says “NOGO” or whatever, it could be almost anything, even for a SAAMI-supported cartridge.

[Disciple]

Insist on gauges with the actual headspace number, so you will know how much less than maximum the No-Go (likely) is.

Field gauges are a relic of the past like proof load testing, when poor designs, questionable material and hardening processes were the norm.

I have a 1.4696 gauge. Is this useful or not?

[gunnerblue]

Just like "mil-spec" . . . . meaningless marketing.

SAAMI does not have a specification for a .300 Blackout headspace gauge, they only specify the chamber dimensions. This is why multiple manufacturer's could have multiple gauges with slight differences, such as tolerancing, finish, material, hardness, etc . . .

Ah...I see what you mean now. I'd guess that if one were to do their due diligence one might find an appropriate set of gauges from certain manufacturers.

[Curlew]

I have a 1.4696 gauge. Is this useful or not?

Sure, it could be. Kinda depends on what you’re trying to figure out.

If a chamber won’t take that gauge, then at least you know it’s not too loose. For comparison, SAAMI max chamber for 223 Rem is 1.4736, so if your gauge doesn’t go, then you’re well under that limit. And as I understand it, Colt’s max chamber for a new M16 is 1.4706, and so you’re under that too. (But the chamber might be too short.)

If the gauge does go, then you know the chamber is on the looser side, but you don’t know by how much, just that it’s longer than 1.4696, maybe by a little or maybe by a lot.

[T2C]

Just like "mil-spec" . . . . meaningless marketing.

SAAMI does not have a specification for a .300 Blackout headspace gauge, they only specify the chamber dimensions. This is why multiple manufacturer's could have multiple gauges with slight differences, such as tolerancing, finish, material, hardness, etc . . .

Which gauges, other than .300 Blackout headspace gauges, should we be concerned about?

[lysander]

Thanks — I had missed that section. It does give a blueprint for the gauge, but as far as I can see SAAMI remains essentially silent on the question of what length dimensions to use for “GO” and “NOGO” (never mind “FIELD”). They just have to be somewhere between chamber MIN and MAX.

So e.g. if I wanted to emulate what Sam Colt and Uncle Sam use for the M16 chamber, I could make a pair of gauges per the Z299.4 print, one with F=1.4646 and another with F=1.4706, labeled “Sams GO 223 Rem” and “Sams NOGO 223 Rem” respectively, and that would comply with SAAMI guidance.

My point is basically to echo Todd.K: if the gauge doesn’t have a length marked on it, it’s hard to know what you’ve got. If it’s a military gauge made to a specific drawing, great. But if it’s a commercial gauge that just says “NOGO” or whatever, it could be almost anything, even for a SAAMI-supported cartridge.

SAAMI MIN is the shortest chamber that SAAMI feels is safe from pinching the case neck and causing overpressure.

SAAMI MAX is the longest chamber SAAMI feels is safe from case separation, akin to the the military FIELD.

"GO" and "NOGO" are manufacturing limits and are usually decided on for maximum accuracy, longevity, and acceptable tolerances for the price point. "NOGO" being the maximum the factory will send out the door on new production. This intermediate gauge length is actually of little use to the average shooter, because you (the shooter) may not have access to all the information behind the choice of factory headspace dimensions. For example, if I shoot thousands of rounds in developing my new rifle and find that best accuracy is at a headspace of 1.4640" and 1.4710", and so I make barrels that headspace out to GO = 1.4640", and NOGO = 1.4710". Of course, Bubba will immediately get on the internet and complain that some barrels will not accept a USGI M16 MIN (1.4646"), and some barrels are above USGI M16 NOGO limit (1.4703"), never mind the fact that they are well inside the SAAMI limits (1.4636 - 1.4736") and M16 FIELD (1.4730"), so safe.

Generally, all the shooting public needs is the SAAMI MAX and MIN for any caliber, and if you build your own barreled actions, or match bolts, you probably want a set that goes from MIN to MAX in 0.001" steps, so you can actually measure your headspace to within a thousandths of and inch.

USGI M16 headspace gauges are actually pointless in my opinion. The entire range of USGI M16 gauge lengths fall inside the SAAMI safe limits.

In short, don't over complicate things.

[Curlew]

Nice explanation, and with one possible exception, I think we’re seeing eye-to-eye on this stuff.

USGI M16 headspace gauges are actually pointless in my opinion.

I have no insight into how Colt chose the M16’s GO and NOGO dimensions, but it doesn’t seem crazy to me to put GO a little bigger than MIN to maybe help accommodate some crud in the chamber, or to put NOGO a little smaller than MAX to allow for wear. I wonder though whether Colt even uses such a gauge in practice — based on something you’ve posted previously, it sounds like they might actually gauge the barrel assembly and bolt separately.

[1168]

you probably want a set that goes from MIN to MAX in 0.001" steps, so you can actually measure your headspace to within a thousandths of and inch.

Is there a set you recommend?

[lysander]

Nice explanation, and with one possible exception, I think we’re seeing eye-to-eye on this stuff.



I have no insight into how Colt chose the M16’s GO and NOGO dimensions, but it doesn’t seem crazy to me to put GO a little bigger than MIN to maybe help accommodate some crud in the chamber, or to put NOGO a little smaller than MAX to allow for wear. I wonder though whether Colt even uses such a gauge in practice — based on something you’ve posted previously, it sounds like they might actually gauge the barrel assembly and bolt separately.

They do.

This for the assembled barrel and extension:


And, this for the bolt

[lysander]

Is there a set you recommend?

The usual US makers.