Need verification on Headspace gauges

[jjohnson636]

I need verification on whether I'm looking at the correct headspace gauges for 300 win mag action
http://www.midwayusa.com/find?userSe...e+.535%22+Base

Now I've been doing some reading on headspacing and could use a little clarification. If I will only use factory ammo I should be fine checking headspace with the gauges listed in the link, but if I were to hand load (which I was strongly considering at some point) would I have to use another method of checking for correct headspace? All the videos and reading material I've read so far only used the go and no go gauges for headspacing, so exactly what is the field gauge use for?

[barrel-nut]

Your link does not lead to headspace gauges on my phone, just a sorta generic Midway screen. Probably my phone's fault. For a belted magnum, the headspace gauges are short and stubby, as they space off of the belt, not the shoulder. Set yyour barrel using the go- and no-go gauges. Then you should be good for any factory ammo, as you stated. Should you decide to handload, you would use brass fired in your rifle, and set your sizing die to just barely bump the shoulder on the fired brass. No need to change anything on the gun. After firing, you would essentially be treating your brass as if it now headspaced off of the shoulder instead of the belt. This would minimize case stretching, and somewhat prolong brass life.

[jjohnson636]

So measuring the fired casing concerns prolonging the use of the brass not measure headspace of my action correct? I'll have to search for more in-depth videos and article pertaining to brass loading procedures.

[barrel-nut]

If I'm understanding your statement correctly, yes.
You need the two gauges on the left in the picture.

[LongRange]

yes those are the right ones...most belted mags are the same except for the 240 weatherby...belted mags are based off the 375 H&H.

[44magleo]

Are you installing a 300 win barrel or do you just want to check your headspace?
If your rifle already has the barrel installed by the factory or a good gun smith, then the headspace is ok.
It might be cheaper to just take your rifle to a gunsmith and let him check the headspace.
Leo

[Robinhood]

Barrel Nut has given some good advice. If you understand

set your sizing die to just barely bump the shoulder

you are better off. Or you could just use a lee collet die.

[44magleo]

There are headspace gauges that check the rifles head space, and there are gauges that check the head space of a cartridge.On belted cases the cartridge headspaces in the rifle on the belt. This holds the cartridge back against the bolt face. Now a lot of belted cases are a bottle neck case, before firing the shoulder on the case is a loose fit to the chamber. After firing the should gets fire formed to match the chamber. Now when you reload you adjust the sizing die, you set it so the die just touches the shoulder. This lets the case headspace on the shoulder instead of the belt. This prolongs case life. If you adjust the die tighter it pushes the shoulder back to factory specs, leaving a bit of space between the shoulder and chamber. This push back and fire forming reduce case life. Best to adjust for minimal head space between shoulder and chamber. Of those gauges the first, the go gauge you should be able to close the bolt on it. The no go gauge is made a bit longer and you should not be able to close the bolt. The field gauge is even longer than a no go gauge. It was designed as a tool to determine if the field if a military rifle was still fit to use. If your rifle won't close on the field gauge it is still safe to use but the head space is too large. If you can close the bolt on the field gauge your rifle isn't safe, the head space is excessive. As I said, if your barrel is factory fitted or installed by a gun smith then the rifle headspace is ok. You don't need any of those gauges. When using factory ammo your rifle is good to go. When reloading just keep the sizing die adjusted to just touch the shoulder of fired cases and your reloads will be fine. Most reloading manuals explain this very well. Get several and read all of them. Leo

[Hotolds442]

the go gauge you should be able to close the bolt on it. The no go gauge is made a bit longer and you should not be able to close the bolt. The field gauge is even longer than a no go gauge. It was designed as a tool to determine if the field if a military rifle was still fit to use. If your rifle won't close on the field gauge it is still safe to use but the head space is too large. If you can close the bolt on the field gauge your rifle isn't safe, the head space is excessive.

Sorry Leo, your description of the field gauge is inaccurate. It is between the go and the no-go in length

Go Gage:Ascertains correct headspace dimensions by duplicating the minimum allowable distance, from the case head (or boltface) to the datum line of the cartridge's shoulder, allowed within the chamber to secure a loaded cartridge and allow the breech to be closed in a safe manner for firing.
If the firearm locks up in battery when the gage is inserted, then it is within acceptable minimum headspace dimensions and may be used.

No-Go Gage:

Duplicates the distance over the maximum allowable, from the case head (or boltface) to the datum line of the cartridge's shoulder, allowed within the chamber to secure a loaded cartridge and allow the breech to be closed in a safe manner for firing.

If the firearm locks up in battery when the gage is inserted, then it is not within acceptable maximum headspace dimensions. The firearm should not be used and should be examined further by a knowledgeable gunsmith.

Field Gage

Duplicates the maximum allowable distance, from the case head (or boltface) to the datum line of the cartridge's shoulder, allowed within the chamber to secure a loaded cartridge and allow the breech to be closed in a safe manner for firing.

If the firearm locks up in battery when the gage is inserted, then it is at the maximum acceptable headspace and should be examined further by a knowledgeable gunsmith.

[44magleo]

Thanks. I guess I was told wrong about the field gauge. Now I know.
Leo

[thomae]

Disclaimer: I have never used a field gauge, so I never thought too much about it, but the below information is what I always "thought" was true.

According to Brownell's web site, a field gauge Measures "The Largest Safe Dimensions:"
Field gauges are essential tools for ensuring a safe chamber on used rifles. Slightly longer then a No-Go gauge, the Field gauge is used to measure the absolute maximum allowable, safe chamber headspace. If the bolt closes on a No-Go gauge, the Field gauge is used for an additional check. If the bolt then closes on the Field gauge, the rifle is considered unsafe to shoot, you’ll have to have the barrel set back and rechambered.

According to Wikipedia (I know, not the MOST authoritative of sources):
The below table illustrates the function of each gauge:

Gauge	Measurement	If Bolt Closes on Gauge	If Bolt Does Not Close on Gauge
Go	Minimum factory-spec spacing	"Go". Headspace is greater than minimum spec.	"No-Go." Headspace is too short. Bolt will not close on factory-spec cartridges, resulting in jammed brass, and firing out-of-battery, both dangerous conditions.
No-Go	Maximum factory spec spacing	"No-Go." Headspace is greater than the maximum factory spec. Risk of case ruptures if used.	"Go". Headspace is below maximum factory spec.
Field	Absolute maximum safe spacing.	Rifle is unsafe to fire. Headspace is greater than what is considered safe to fire. High risk of case rupture if fired.	Generally "go". Headspace is below the maximum for what is considered safe to fire when using factory spec ammunition. Rifle should be evaluated by a gunsmith for possible re-headspacing.

[barrel-nut]

Thanks, Thomae. The Brownell's explanation is good and the easiest to follow.
For my purposes, if it closes on a no-go, it's too much, and I'll fix it as necessary.
I do (now) understand the purpose of the field gauge, especially in military applications.

[Trapper458]

This is a good explanation taken from the CMP website:

The three gauges you will encounter are the “GO”, “NO GO” and “FIELD”. CMP only uses “GO” and “NO GO” gauges but I will describe all three.

The “GO” gauge - is most commonly used when installing a new barrel and reaming the chamber to size. The bolt should fully close on the “GO” gauge, if it fully closes you can be sure you have enough room in the chamber to prevent the cartridge from being crushed during chambering. The “GO” gauge can also be thought of as a minimum safe headspace gauge and the rifle's bolt must be able to fully close with it in the chamber.
The “NO GO” gauge - is used to make sure a firearm does not have excessive headspace. The bolt should NOT fully close on the “NO GO” gauge, if the bolt cannot be closed on the “NO GO” gauge then you know your rifle does not have headspace that is excessive. The “NO GO” gauge can be thought of as a maximum headspace gauge and should not be able to fit in the rifle's chamber with the bolt fully closed. If the bolt DOES close on the “NO GO” gauge, it does not necessarily mean that the rifle is unsafe; it does however show that a further check with the “FIELD” gauge would be necessary to determine if it is safe to shoot.
The “FIELD” gauge - is used to check absolute maximum headspace. If the bolt closes fully on the “FIELD” gauge the rifle IS NOT to be fired and should be considered unsafe to shoot. CMP does not use this gauge because rifles that pass the “FIELD” check but fail the “NO GO” are approaching the point where they will be unsafe to shoot. Our standard for maximum headspace is the “NO GO” gauge to ensure our customers will be able to shoot safely for many years.

Regards,
Trapper458