Head space aftr barrel change.

[KMW 1954]

Just finished changing barrels on a Savage 110 with a Shilen Match Select barrel from NSS and used a PTG head space gauge to set it. After finishing there is a huge difference between what was and what is now.

This is a 6ARC and the datum spec I see is 1.190" with a tolerance of -.007". With the original barrel I was full length sizing my brass to 1.190" measured with a Hornady comparator. Now with the new barrel I need to size this brass down to 1.1850" to get it to where the bolt will close. When measuring the gauge with the Hornady comparator I get 1.1835"

So 2 questions. From others experience is this gauge on the short side? Is there an advantage to a short head space as opposed to the longer head space which I had originally?

 

[memilanuk]

The gauges are usually less the issue than the comparators that we as end users have ready access to.

Comparators are great and immensely useful, but the biggest thing to realize and remember is that they are only *relative* measurements, not *absolute*.

I'm going to go out on a limb here and say that you can take nearly any comparator, clamp it on any headspace gauge, and you're not going to get the same measurement as what's marked on the side of the HS gauge.

The Hornady (and others like it) have a hole that is a specific size to match up with the datum line at the midpoint of the shoulder as shown on a cartridge or chamber print. For a .308 Win (and family) that datum line is described by a circle that is 0.400" in diameter. For a .223 Rem (and family) it's 0.330" in diameter. Not sure what it is for a 6 ARC, but you get the idea.

So the Hornady comparator for that cartridge family has a hole that size, so it should match the 0.400" line (for example, since I 'think' in .308 Win) on the shoulder of the headspace gauge, right? Not so much. One, it's aluminum. Two, they put a small radius on the edge of that hole, to help with durability. But... the result is that it doesn't read *exactly* at the 0.400" line any more. If I take *my* Hornady comparator and clamp it on my Forster headspace 'GO' gauge... it reads 1.624". Not 1.630", which is what it says on the side. Similarly, if I clamp it on the 'NO-GO' gauge, it reads... 1.628", not 1.634".

The *relative* difference - 4 thou - is spot on. The *absolute* reading... is irrelevant.

Other comparators (SAC, 419, Sinclair before that) take a different approach, and use an insert that has a beveled hole which matches the shoulder angle of the cartridge family (in theory). In that case, the numbers you get will absolutely bear not one iota of resemblence to what's printed on the headspace gauge... but you can zero your calipers with the comparator on that gauge, and get *relative* measurements to work off of.

the datum spec I see is 1.190" with a tolerance of -.007". With the original barrel I was full length sizing my brass to 1.190" measured with a Hornady comparator.

^this... might have been a mistake. You should have been sizing your brass to some value relative to it's fired state i.e -2 or -3 thou, not to some arbitrary number that you read some where.

When measuring the gauge with the Hornady comparator I get 1.1835"

...is this gauge on the short side?

I'd say the odds are better that the headspace gauge is spot on, and your comparator is reading exactly as it should.

 

[Walt Krafft]

Sounds like your first barrel was set close to the nogo gage. Did you check it? If your Savage is using the barrel nut, you could set it out to use your ammo as is.

 

[KMW 1954]

The gauges are usually less the issue than the comparators that we as end users have ready access to.

Comparators are great and immensely useful, but the biggest thing to realize and remember is that they are only *relative* measurements, not *absolute*.


^this... might have been a mistake. You should have been sizing your brass to some value relative to it's fired state i.e -2 or -3 thou, not to some arbitrary number that you read some where.


I'd say the odds are better that the headspace gauge is spot on, and your comparator is reading exactly as it should.

Yes I do understand one is a Comparator and not a measurement gauge. Still both should measure to the same diameter and then be compared to one another. Which in this case give a value that one is .004" shorter than the other. Hence the old sized brass will not chamber. Too long.

The old brass measurement was sized after firing in the rifle using the same comparator. Measured before firing and then after firing. In this case I was using a minimal shoulder bump and basically just removing the case bulge and resizing the neck. Never looked at a print before a couple days ago.

As for the head space tool, it is withing spec so I am not afraid to use it. Just that now I am going to have to resize about 50finished pieces and resize another 50 that are sitting waiting.

Still the last question, is there any advantage to the finished length being longer or shorter other than the amount of work hardening from sizing shorter?

 

[gunsandgunsmithing]

Yes I do understand one is a Comparator and not a measurement gauge. Still both should measure to the same diameter and then be compared to one another. Which in this case give a value that one is .004" shorter than the other. Hence the old sized brass will not chamber. Too long.

The old brass measurement was sized after firing in the rifle using the same comparator. Measured before firing and then after firing. In this case I was using a minimal shoulder bump and basically just removing the case bulge and resizing the neck. Never looked at a print before a couple days ago.

As for the head space tool, it is withing spec so I am not afraid to use it. Just that now I am going to have to resize about 50finished pieces and resize another 50 that are sitting waiting.

Still the last question, is there any advantage to the finished length being longer or shorter other than the amount of work hardening from sizing shorter?

Yes, I know the pros and cons to this but what if you use a fired case as your hs gage, or maybe a layer or so of tape on the go gage, enough to make it the same length as your fired case..and set the bbl by that? Too late to know now but it's possible or even likely that something moved a bit when the first bbl was put on using the gage. Hence your longer brass.

 

[Walt Krafft]

Still the last question, is there any advantage to the finished length being longer or shorter other than the amount of work hardening from sizing shorter?

No advantage or disadvantage. There is a tolerance between the GO and NOGO.

 

[KMW 1954]

Thanks all. Not a lot of experience doing this and enjoying what I'm learning. Much of this is still part of the learning process which brings the questions.

The thought of using a fired or even a new case as a head space gauge has been considered but wasn't certain if this was an acceptable practice. Still what amazes me is these 2 rifles at complete opposite ends of the tolerance spectrum.

 

[LVLAaron]

Any chance you've got a field gauge?

 

[gunsandgunsmithing]

Thanks all. Not a lot of experience doing this and enjoying what I'm learning. Much of this is still part of the learning process which brings the questions.

The thought of using a fired or even a new case as a head space gauge has been considered but wasn't certain if this was an acceptable practice. Still what amazes me is these 2 rifles at complete opposite ends of the tolerance spectrum.

I would not suggest using a new case(short). Your situation simply calls for duplicating the headspace between your two bbls. A fired case does that, very closely but just be careful, as you can crush an empty case a bit fairly easily and still end up a bit short. You can fill a fired case with epoxy but it's probably easier to just tape the end of your go gage or possibly just use your no-go, due to it being .004 longer(I think). Check it. At this point, it is what it is.

 

[KMW 1954]

I would not suggest using a new case(short). Your situation simply calls for duplicating the headspace between your two bbls. A fired case does that, very closely but just be careful, as you can crush an empty case a bit fairly easily and still end up a bit short. You can fill a fired case with epoxy but it's probably easier to just tape the end of your go gage or possibly just use your no-go, due to it being .004 longer(I think). Check it. At this point, it is what it is.

Just as I suspected about using a brass case. I believe I am going to just leave it as-is and be happy. Just add this to my learning curve. All this does answer most of those questions floating around my mind!

 

[Coyotefurharvester]

I've used fired/sized brass for HS go gage, filled it with 2 part 24 hr epoxy. Wear gloves it will heat up quickly, no worries about crushing the brass after it hardens. For a rifle that will NEVER see factory rounds.

 

[DR_2297]

unless you have a way to qualify the diameter that the comparator is using as it’s registering point you have no way to know if it is good or bad or anywhere in between.

If you have a go gauge from a reputable manufacturer assume it is correct, within tolerance, and use it as your master to zero your comparator on your calipers.

Or if you want a sense check buy another go gauge from an different mfg and compare the two…

But going off of a dimensioned drawing with an unqualified gauge rarely pencils out. Especially a theoretical datum line.

Do you have a case gauge? Is the new brass short in it? Is the old brass long? For things like ARs I FL size to HS go gauge. For bolt guns - especially single shot - I will use fired cases as the zero and bump shoulder back .002-.003 while FL sizing.

 

[KMW 1954]

Do you have a case gauge? Is the new brass short in it? Is the old brass long?

Yes I have one from Hornady. Both are within the gauges min and max. The old loads are right even with max and even with the top of the gauge after FL sizing. The new load is just above the min level in the gauge and that is after sizing the old brass to where the bolt will close.