How to accurately measure (not validate) headspace?

[Edd]

Thx, Mram. From this 6.5 PRC drawing, SAAMI says the headspace should be 1.6486" - 1.6586", or a full 100 thousandths of tolerance. Some responders have said it's not necessary to know headspace, just make sure it's within the safe range with go/no-go gauges, but it seems that view misses several points: 1) for precision, we try to load to a specific bullet jump that our rifle likes, 2) when we're sometimes testing the difference between a 0.010" - 0.020" jump, SAAMI's 100 thou tolerance is way too large for our purposes, we need to know the actual chamber dimension, and 3) for custom barrels, the SAAMI spec doesn't relate to custom chambers cut with custom reamers. These are the reasons that I think we need to know our chamber's dimensions, rather than just validating safety with go/no-go gauges. Otherwise, we're working blind instead of working from data, and in the quest for precision, data is king.

The SAAMI Headspace tolerance is 10 thousandths not 100 thousandths. Knowing what the number is will do you no good because your measurement will not match that number. For reloading, you create your own number and size your cases to your number. The advantage of using a headspace gauge to determine your number is because it is rigid enough to not change its dimension. You need to determine what your chamber length is compared to a go gauge then compare the length of your sized brass to that length.

 

[MNbogboy]

Thx, Mram. From this 6.5 PRC drawing, SAAMI says the headspace should be 1.6486" - 1.6586", or a full 100 thousandths of tolerance. Some responders have said it's not necessary to know headspace, just make sure it's within the safe range with go/no-go gauges, but it seems that view misses several points: 1) for precision, we try to load to a specific bullet jump that our rifle likes, 2) when we're sometimes testing the difference between a 0.010" - 0.020" jump, SAAMI's 100 thou tolerance is way too large for our purposes, we need to know the actual chamber dimension, and 3) for custom barrels, the SAAMI spec doesn't relate to custom chambers cut with custom reamers. These are the reasons that I think we need to know our chamber's dimensions, rather than just validating safety with go/no-go gauges. Otherwise, we're working blind instead of working from data, and in the quest for precision, data is king.

Edd is 100% correct in the above post. The saami number will never match your comparator number.
Each chamber in each individual rifle will have its own baseline when creating brass and loaded ammunition.
Also actual chamber headspace has no relationship to seating distance. Seating distance to the lands is generated from a measurement from bolt face to the lands themselves. Case headspace (chamber headspace + case clearance to bolt face) will affect seating distance if not controlled in sizing operations.
Sometimes seating distances as small as .002 or .003 can be noticed in precision loading. Therefore case headspace is an important control in achieving consistent distance of bullet ogive to the lands.

Makes a person think that not many use Saami numbers or "actual" chamber headspace numbers when developing precision ammunition.
We need to know how long our brass should be from base to shoulder and how deep to seat our bullets measured from base to ogive.

 

[Muddyboots]

Throwing this to wall to see if it sticks. I remove firing pin shroud from 700 bolt so it is free and clear floppy bolt throw with zero resistance on closure as if you are setting the barrel headspace with a go-gauge. You have removed any false positives from feeling any resistance to bolt closure after you have resized a brass. With fired brass that is measured with whatever tool you choose to get a "headspace" measurement of the brass. Put in rifle to confirm there is resistance to closure. Using Redding Competition Shellholder Set, select 0.002 shell holder and resize the brass without changing the die setting itself. Measure brass again and should be 0.002 setback. Place into chamber, close bolt. If still resistance, remove and change shellholder out to 0.004 and resize. Eventually you will find shellholder measurement that will give you the bump back to allow bolt closure without resistance. This measure will be pretty darn close to the actual headspace measurement. Just a method I have used to get basic measurement of a rifle.

Link: Redding Competition Shellholder Set

 

[Mram10us]

Thx, Mram. From this 6.5 PRC drawing, SAAMI says the headspace should be 1.6486" - 1.6586", or a full 100 thousandths of tolerance. Some responders have said it's not necessary to know headspace, just make sure it's within the safe range with go/no-go gauges, but it seems that view misses several points: 1) for precision, we try to load to a specific bullet jump that our rifle likes, 2) when we're sometimes testing the difference between a 0.010" - 0.020" jump, SAAMI's 100 thou tolerance is way too large for our purposes, we need to know the actual chamber dimension, and 3) for custom barrels, the SAAMI spec doesn't relate to custom chambers cut with custom reamers. These are the reasons that I think we need to know our chamber's dimensions, rather than just validating safety with go/no-go gauges. Otherwise, we're working blind instead of working from data, and in the quest for precision, data is king.

10 thousandths. 100 thousandths would definitely cause case head separations All go gauge manufacturers will make a Go gauge to 1.6486 for the 6.5prc. They might differ with the no go gauge. .004-.006" difference depending on manufacture and cartridge

 

[Jeffpatton00]

Hmm, processing numbers on the fly.. not the best recipe for accuracy.

 

[MNbogboy]

Hmm, processing numbers on the fly.. not the best recipe for accuracy.

Expand on this..

 

[tobnpr]

On using a case with extruded primer...

If you could do this with a precision ground headspace gauge, it would be better.
With a case, you're getting relative headspace, which is what most of us care about.
This is the same principle as the OAL gauge most of us use.

Case isn't a headspace gauge, and it's for that exact reason we have headspace gauges.

Cutting shim stock, stuck to the base of a headspace gauge with a dab of grease.
Or, Plastigage if the spirit moves ya. Not as fancy, but a piece of soft solder will serve the same purpose.

 

[fguffey]

Just to clarify further, headspace is the distance to the datum point. Example: .400" on a 6.5cm or .420" on a 28 Nosler. Dont confuse with neck shoulder junctions.

To answer your question, take the go gauge (min headspace measurement) and check for a snug fit like you mentioned. Add tape or shim stock and that should get you to +/-.001".

I can check the length of a chamber from the datum to the bolt face 3 different ways without a head space gage. The easiest chamber to check is the chamber in the 03/03A3. I am more interested in offsetting the length of the chamber from the datum to the bolt face with the length of the case from the datum to the case head.

It is claimed I have rifles with excessive head space. For example one of my M1917s chamber is .002" longer than a field reject length gage. That puts the minimum length/full length sized case .016" too short from the shoulder of the chamber to the bolt face. I do not see this as a problem, I form 280 Remington cases to 30/06 cases by adding .014" to the length of the case when forming. Adding .014" to the length of the case from the datum to the case head leaves me a 30/06 case that has .002" clearance between the case head and bolt face (or .002" clearance between the shoulder of the case and shoulder of the chamber). I say that because the fired case proves the shoulder of the case was not against the shoulder of the chamber when fired.
And then there is the problem with finding a reloader or smith that understands what happens when I fire minimum length/full length sized cases (factory/store bought) ammo in the long chamber.

JeffPatton, I can work with you off line, you could be in the neighbor or we could work by phome.

F. Guffey

 

[Mram10us]

I can check the length of a chamber from the datum to the bolt face 3 different ways without a head space gage. The easiest chamber to check is the chamber in the 03/03A3. I am more interested in offsetting the length of the chamber from the datum to the bolt face with the length of the case from the datum to the case head.

It is claimed I have rifles with excessive head space. For example one of my M1917s chamber is .002" longer than a field reject length gage. That puts the minimum length/full length sized case .016" too short from the shoulder of the chamber to the bolt face. I do not see this as a problem, I form 280 Remington cases to 30/06 cases by adding .014" to the length of the case when forming. Adding .014" to the length of the case from the datum to the case head leaves me a 30/06 case that has .002" clearance between the case head and bolt face (or .002" clearance between the shoulder of the case and shoulder of the chamber). I say that because the fired case proves the shoulder of the case was not against the shoulder of the chamber when fired.
And then there is the problem with finding a reloader or smith that understands what happens when I fire minimum length/full length sized cases (factory/store bought) ammo in the long chamber.

JeffPatton, I can work with you off line, you could be in the neighbor or we could work by phome.

F. Guffey

How else do you check headspace accurately? I'm genuinely interested. Ball bearing? As for long headspace past a field gauge, I have had one ai chamber I cut too long and had nothing but issues with separations. Found my screw up was .004" over my target for some reason and was great after it was fixed. All that to say, great that you haven't had these issues, but I am leery of excessive headspace past a nogo. Just me

 

[fguffey]

How else do you check headspace accurately? I'm genuinely interested.

It is not possible to discuss head space methods and techniques on the Internet. Accurately, should be within .001", Again, I can work with you off line.

F. Guffey

 

[Mram10us]

It is not possible to discuss head space methods and techniques on the Internet. Accurately, should be within .001", Again, I can work with you off line.

F. Guffey

Ummmm, ok We have 4 pages on this thread alone, but ok.

 

[misterc01]

I use an RCBS precision mic. Once-fired get me to about .002-.003 over, and I bump it back until I get to within -.002. Everything chambers fine.

 

[fguffey]

Ummmm, ok We have 4 pages on this thread alone, but ok.

We have 20 years of pages and tons of gigs with no answer. Again, I was in a gun parts store when a man walked in with an exotic/beautiful custom looking rifle; he wanted the rifle check for head space. The owner of the fine gun parts establishment informed the owner he could not check the rifle for head space because he did not have a head space gage for that magnificent rifle. I did not get involved, the owner of the rifle left and then I told the owner of the parts store I can check the head space on any rifle 3 different ways without a head space gage.

I know, that would just lock up most smiths/reloaders but not the owner of the fine establishment; he leaned forward and ask; "HOW?". I picked an easy method, he being a good listener and a smith with over 250 head space gages followed me through , when fished he said, "Well I be dammed". I thought that was nice, he is one of those people that can discern fact from fiction and truth from nonsense. And then I offered to convert his go-gages to go-to-infinity gages using the same technique. Problem, he sells gages not an education.

Forgive, I forgot; he is the proud owner of an adjustable head space gage, that would be the one I had never seen before but looks like a few of my Starrett inside micrometers.

F. Guffey

 

[nksmfamjp]

4 pages....didn't read them all, but a GO gauge + plastigage should be a way to get the exact number.

Make sense?

This is not how I do it. I do it the remove innards from bolt and test fit cases FL sized wrong.