Documenting chamber measurements?

[Hannibal]

Saw a discussion elsewhere concerning accurately measuring a chamber, and it occurred to me that I am not aware of any method to document the actual length of a given rifle chamber, one for a bottleneck cartridge. I've seen the Match Rifle Headspace Gauge sets sold by Brownell's, which would essentially be a 'trial and error' process, I presume. (?)
Is there another method that does not involve using a pre-made gauge?

I'm not trying to do anything in particular, this is just to help my own understanding.

Thanks for any input.

[No Blue]

It's called Cerrosafe; a metal alloy that melts at 190 degrees, you plug the bore just as the rifling starts with a cleaning patch. Then pour the liquid metal into the chamber. It'll harden in a couple of minutes and you knock it out with a cleaning rod.

About an hour later, measure it up; it's shrunk down to the correct size.

At Brownell's, you can reuse the casting by melting it again. It's something everybody messing around with guns should have to answer those chamber questions.

[Idz]

If you just want to see what the chamber looks like then use regular canning paraffin wax instead of cerrosafe. It will show any defects and give you a pretty good idea of dimensions. If you need more accurate measurements use the metal.

[Cap'n Morgan]

The correct way to measure the chamber length of a bottleneck cartridge is to have a ball bearing of a known size resting on the angular flats of the chamber shoulder - then measure the distance from the chamber face to the top of the ball. The rest is a matter of trigonometry.

[Hannibal]

I find all 3 of the above mentioned methods to be confusing, or else I am confused.

As I understand it, the length of the chamber for a bottleneck cartridge is the distance between the datum line on the sboulder to the bolt face, correct? I can not envision any method to measure this accurately due to the necessity of the bolt face having to be in place to establish the position of the bolt face.

I understand many people measure a piece/pieces of fireformed brass with a caliper and comparator and add .001" and this will get you darn close, but if the brass is not fully fireformed or unusually soft/hard then close is the best you can get, correct?

I'm not asking about trying to establish a cartridge overall length. I'm trying to ascertain how to measure the distance between the datum line on the shoulder and the bolt face.

If I didn't explain myself thoroughly enough, I apologize. And if I'm missing something obvious, I apologize for that as well. I've searched every way I can think of and there is no way to measure this that I can find, but SURELY it can be done somehow, accurately. To 4 decimal places. (.0000")

Thanks for the input thus far.

[No Blue]

You need the Hornady case gauge deal. It has 5 inserts with various size holes bored in them. Select the insert with the correct size, install it on your dial caliper. The hole size corresponds to the datum dimension.

Measure a piece of fired brass from your rifle and compare the dimension to the SAAMI blueprint.

You can also use a factory new brass, that will tell you how close to the print they're making it.

Then you put a piece of tape, an aluminum shim from a beer can, or if you're a machinist, you have a selection of various thickness of brass shim stock and can use that; that's what I do.

Put the brass with the shim in the rifle and close the bolt. If you can close the bolt, add another shim and try it again. Eventually you'll find out when the bolt won't close.

Measure the brass with the shims and that is your headspace dimension or chamber length. Understand?

Rather than buying $35 headspace gauges for each of my rifles, I use this procedure.

A gunsmith told me the Hornady inserts are cut with a small chamfer at the mouth of the hole. He said for extra accuracy, machine the insert in a lathe to remove it, and you'll have a better gauge. I just use it like it was made. I'm looking for a thou or so accuracy....

Gunsmiths have been using this method for over 120 years, but they didn't have the Hornady tool. They just shimmed some brass and see how it fit....


https://www.midwayusa.com/product/47...ith-comparator

[country gent]

As I understand what your asking is to measure the overall length of the chamber? So that trim length can be determined for a given rifle. Sinclars used to sell a plug that was 2 dia small was caliber specific and fit in a case sung with a larger dia around neck dia. A case was modified by trimming back .100-.150 and a slit put in it to make it a spring collet. This plug was inserted long and chambered pushing it back into the case when it came to bear on the case mouth shoulder. You then removed case and pinched neck tight and measured its overall length. As to a full chamber measurement then the chamber cast with cerrosafe is required. This is only good and accurate for a short time due to age shrinking of the material, but this gives an accurate male image of the chamber that can be easily measured with standard tools. Other wise its measure the reamer that cut the chamber and hope it didn't cut oversize or egg shaped.

[Hannibal]

As I understand what your asking is to measure the overall length of the chamber? So that trim length can be determined for a given rifle. Sinclars used to sell a plug that was 2 dia small was caliber specific and fit in a case sung with a larger dia around neck dia. A case was modified by trimming back .100-.150 and a slit put in it to make it a spring collet. This plug was inserted long and chambered pushing it back into the case when it came to bear on the case mouth shoulder. You then removed case and pinched neck tight and measured its overall length. As to a full chamber measurement then the chamber cast with cerrosafe is required. This is only good and accurate for a short time due to age shrinking of the material, but this gives an accurate male image of the chamber that can be easily measured with standard tools. Other wise its measure the reamer that cut the chamber and hope it didn't cut oversize or egg shaped.

I am trying to determine overall chamber length, but not for the purpose of establishing trim length. Let's say you or I decided we wanted to set the shoulder .0015" shorter than the overall chamber length when resizing fireformed brass. Not that I do, just as an example. Without knowing the precise overall chamber length, this is not possible to do accurately, correct?

Using your cerrosafe example, I can see how the depth of the chamber can be determined, and then the distance from the end of the shank to the bolt face would have to be added to have the overall chamber length, correct?

And am I wrong, or would a comparator have to be used to measure either the chamber cast or the reamer? Which I understand is not considered to be a highly accurate measurement. (?)

Perhaps THAT is my question. How can the position of the datum line on the chamber shoulder be located/established so that a precise length measurement of the chamber can be taken?

I am sorry, I know of no simple way to phrase my question. And I thank you for your reply. I appreciate any help/light that can be shed on this.

[Hannibal]

You need the Hornady case gauge deal. It has 5 inserts with various size holes bored in them. Select the insert with the correct size, install it on your dial caliper. The hole size corresponds to the datum dimension.

Measure a piece of fired brass from your rifle and compare the dimension to the SAAMI blueprint.

You can also use a factory new brass, that will tell you how close to the print they're making it.

Then you put a piece of tape, an aluminum shim from a beer can, or if you're a machinist, you have a selection of various thickness of brass shim stock and can use that; that's what I do.

Put the brass with the shim in the rifle and close the bolt. If you can close the bolt, add another shim and try it again. Eventually you'll find out when the bolt won't close.

Measure the brass with the shims and that is your headspace dimension or chamber length. Understand?

Rather than buying $35 headspace gauges for each of my rifles, I use this procedure.

A gunsmith told me the Hornady inserts are cut with a small chamfer at the mouth of the hole. He said for extra accuracy, machine the insert in a lathe to remove it, and you'll have a better gauge. I just use it like it was made. I'm looking for a thou or so accuracy....

Gunsmiths have been using this method for over 120 years, but they didn't have the Hornady tool. They just shimmed some brass and see how it fit....


https://www.midwayusa.com/product/47...ith-comparator

I've never handled or seen a comparator set like this, so I don't know for certain, but I've heard comparator sets were fine for darn close measuring, but not highly accurate measurements.
I guess I want to know how the manufacturers of the gauges go about verifying their gauges are correct?
Yeah, I know. 'Who cares?!' Well, I do, actually.

[Hannibal]

The correct way to measure the chamber length of a bottleneck cartridge is to have a ball bearing of a known size resting on the angular flats of the chamber shoulder - then measure the distance from the chamber face to the top of the ball. The rest is a matter of trigonometry.

Think I've just had an epiphany. IF one has a ball bearing of a diameter that matches the diameter of the datum, then a depth mic can be used to measure the distance from the end of the barrel stub to the ball bearing. Then add 1/2 of the ball bearing diameter to the depth mic measurement and viola, an accurate chamber depth measurement.

Sound correct, or am I off in the weeds?

[country gent]

One other way is that is done quite often is to make a gage with the datum line dia sized hole in it to a depth t clear the length of the cartridge forward. This can then be used to measure from base to shoulders datum line. As to the comparators they can be quite accurate depending on operator and magnification. At 100x a .001 variance shows up as .100 in length. The best and how most makwers of heaspace gages measure is Coordinate measuring machines that can be accurate to millionths of an inch. An expensive large piece of equipment that works off of a granite surface plate and mesure with an electric porbe to sense "touch" these are completely hands off measuring with a computer controlled movement and sensing. Probes come in several shapes and sizes. With the comparator measuring can be done accurately on a standard grid, find center of part and drop table half of datum dia bring to touch shoulder and zero x then move table to back of the case head and you have the reading and direct measurement. The special anvil is a comparative measurement that shows the difference between the standard and the part. If you come up with the constant number then the measurement minus the constant for the anvil will be very close also. A CMM is around $60,000 and requires a climate controlled environment. A used optical comparator can be picked up for around $800- &1000 and needs to be leveled up and a good power source and light. The anvil can be made fairly cheap compared to the others with a headspace gage it can be made to be direct reading by using a 1" bigger mic than the dimension calls for. IE If the headspace measurement is 2" then a 3" mic would be used with the gage. For the comaparative measurements a nut 1" across flats with the datum size hole in center of a flat will give good repeatable measurements. It boils down to what you want, How accurate you want and what your willing to spend

[KenH]

I know this isn't the OP's question, but as a way to measure headspace, how about inserting a GO gauge in chamber, then placing a strip of PLASTIGAUGE on head, close bolt, open bolt then check PLASTIGAUGE to see how much headspace is there.

[Hannibal]

I know this isn't the OP's question, but as a way to measure headspace, how about inserting a GO gauge in chamber, then placing a strip of PLASTIGAUGE on head, close bolt, open bolt then check PLASTIGAUGE to see how much headspace is there.

Based upon my past experience with Plastigauge, the trouble would be the turning motion required to close a bolt. I should think soft lead solder would work just fine.

That said, I believe this would be the simplest solution to finding chamber length as the gauges I have are marked with their lengths.

Still requires the use of a gauge and the faith in the manufacturer of that gauge that the length stated is correct, which is not something I would worry about under nearly all circumstances.

[Hannibal]

One other way is that is done quite often is to make a gage with the datum line dia sized hole in it to a depth t clear the length of the cartridge forward. This can then be used to measure from base to shoulders datum line. As to the comparators they can be quite accurate depending on operator and magnification. At 100x a .001 variance shows up as .100 in length. The best and how most makwers of heaspace gages measure is Coordinate measuring machines that can be accurate to millionths of an inch. An expensive large piece of equipment that works off of a granite surface plate and mesure with an electric porbe to sense "touch" these are completely hands off measuring with a computer controlled movement and sensing. Probes come in several shapes and sizes. With the comparator measuring can be done accurately on a standard grid, find center of part and drop table half of datum dia bring to touch shoulder and zero x then move table to back of the case head and you have the reading and direct measurement. The special anvil is a comparative measurement that shows the difference between the standard and the part. If you come up with the constant number then the measurement minus the constant for the anvil will be very close also. A CMM is around $60,000 and requires a climate controlled environment. A used optical comparator can be picked up for around $800- &1000 and needs to be leveled up and a good power source and light. The anvil can be made fairly cheap compared to the others with a headspace gage it can be made to be direct reading by using a 1" bigger mic than the dimension calls for. IE If the headspace measurement is 2" then a 3" mic would be used with the gage. For the comaparative measurements a nut 1" across flats with the datum size hole in center of a flat will give good repeatable measurements. It boils down to what you want, How accurate you want and what your willing to spend

I see. It appears I need to educate myself about comparators more. Obviously, a coordinate measuring machine as you've mentioned is going to be out of the question for me.
Could you please elaborate on the gauge with the datum line diameter sized hole you mentioned at the beginning of your reply? I've read, re-read and thought it over about 4x now, and I can't picture what you are speaking of.

[KenH]

I'd not thought of that, I would expect the twisting motion of bolt when locking just might smear the plastigauge a tad making it hard to get an exact measurement.

[Hannibal]

I'd not thought of that, I would expect the twisting motion of bolt when locking just might smear the plastigauge a tad making it hard to get an exact measurement.

I made that exact mistake checking main bearing clearance once. Otherwise, I would have never thought of it, either. 'Do NOT rotate that crank, any measureable amount, once the plastigauge is in there.'

I never forgot.

[country gent]

Most sammi drawing give the dia of the shoulder at the datum point a piece of steel with this sized hole drilled and bored square to parallel surfaces acts as a "Gage or new anvil for the mic giving a reading from the datum point to the case head. If made and set up with the headspace gauge to read 1.0000 over the headspace gauges marked size then a 1" bigger mic can be used for direct reading other wise a "constant" number is needed. This number is the difference between the gauges marked size and the mics actual reading. Like measuring a thread with thread wires the constant number bring thereading over the wires between the radiuos and vee to a actual measurement. If you have the thread and are making a new one then the comparative measurement from the existing thread to the one your cutting over the wires with out the constant number works. The above gage is a simple piece of steel 3/8" thick 1/2"-3/4" wide and 1" tall with the hole drilled in the center of the 3/8" thick area to a depth to clear shoulder and neck plus so free area but not thru ( have to have a solid area on top for mikes anvil to bear). Ideally start with 1"+ and work in with measurements over the headspace gage to where it reads the same +1" so a 1" bigger mic can be used and direst reading. This takes a little playing to get. Measure and mill surface with in .005+ and then grind or lap in to dead zero. If you decide to stamp the use on the side then do it before working in as stamping will stretch the metal and change size. Engraving is better for this. A piece of gage stock works well here and if desired can be hardened even.

[No Blue]

Most sammi drawing give the dia of the shoulder at the datum point a piece of steel with this sized hole drilled and bored square to parallel surfaces acts as a "Gage or new anvil for the mic giving a reading from the datum point to the case head. If made and set up with the headspace gauge to read 1.0000 over the headspace gauges marked size then a 1" bigger mic can be used for direct reading other wise a "constant" number is needed. This number is the difference between the gauges marked size and the mics actual reading. Like measuring a thread with thread wires the constant number bring thereading over the wires between the radiuos and vee to a actual measurement. If you have the thread and are making a new one then the comparative measurement from the existing thread to the one your cutting over the wires with out the constant number works. The above gage is a simple piece of steel 3/8" thick 1/2"-3/4" wide and 1" tall with the hole drilled in the center of the 3/8" thick area to a depth to clear shoulder and neck plus so free area but not thru ( have to have a solid area on top for mikes anvil to bear). Ideally start with 1"+ and work in with measurements over the headspace gage to where it reads the same +1" so a 1" bigger mic can be used and direst reading. This takes a little playing to get. Measure and mill surface with in .005+ and then grind or lap in to dead zero. If you decide to stamp the use on the side then do it before working in as stamping will stretch the metal and change size. Engraving is better for this. A piece of gage stock works well here and if desired can be hardened even.

You must be an "The Ohio State Univerary" fan; make about as much sense as those clowns....I'm from that school up north....

[Ballistics in Scotland]

The correct way to measure the chamber length of a bottleneck cartridge is to have a ball bearing of a known size resting on the angular flats of the chamber shoulder - then measure the distance from the chamber face to the top of the ball. The rest is a matter of trigonometry.

Exceedingly complicated spherical trigonometry, a subspecies which always eluded me. It could be a very good way of comparing chamber with chamber, if the rear face of the barrel is the same all around, and best if the barrels were detached, so that a depth micrometer rather than the rear end of a caliper could be used.

I think the early part of this thread looked, perhaps erroneously, at two different things. Cerrosafe is the classic material for making a chamber cast, and I don't believe anyone ever ran into trouble or significant inaccuracy by using an aged cast. Car body filler is good, although you feel a fool if you don't oil the chamber first. Melted sulphur makes an excellent cast, but if only slightly overheated it can ignite with a low blue flame, a bit like St. Elmo's Fire in "Moby Dick". I don't believe there is any way it can turn into a larger conflagration, but anything of steel nearby needs to be washed and oiled, or it will rust as condensation forms your own private version of acid rain - and there is something very important of steel nearby.

I think, though, the OP was really making a well educated and thoughtful inquiry about the difficulty of finding numerical specifications of headspace for different cartridges. I believe the reason is that a figure in inches can mean any of several different things, and a person can get himself into serious trouble by misapplying it.

A headspace figure can be the distance from the bolt face to the theoretical junction of the slightly conical chamber body. I say theoretical because chamber reamers normally have slightly radiused corners, and that junction is actually inside the solid steel of the barrel. I don't believe anybody has used the junction of shoulder and neck, which for much the same reason would be hovering in air, and moreover would be altered by different neck diameters. People have, however, specified the headspace as being to a point on the shoulder mid-way between these non-existence points. A better system is to specify headspace to a point where the shoulder comes to some specific diameter.

Some cartridge designs have even shifted from one form of specification to another, with no change in the chamber intended. Thank goodness Roy Weatherby, whose case shoulder shape went from large convex radius about meeting large concave radius, didn't go rimless with it.

We are generally advised against chambering a rifle on the basis of a cartridge case, fired elsewhere or unfired. But for belted and rimmed cartridges I have done it, with a .002in. copper foil shim adhering to the head of several new Winchester cases, and I found the result accurately chambered for several brands.

I'm sorry about the picture. I just couldn't stop myself.

[Cap'n Morgan]

Think I've just had an epiphany. IF one has a ball bearing of a diameter that matches the diameter of the datum, then a depth mic can be used to measure the distance from the end of the barrel stub to the ball bearing. Then add 1/2 of the ball bearing diameter to the depth mic measurement and viola, an accurate chamber depth measurement.

Sound correct, or am I off in the weeds?

If the ball matches the diameter of the datum line, its center will always be above the datum in an angled (conical) hole. The exact amount is depending on the angle - I'm afraid trigonometry must be applied here...

B.I.S.:
My avatar is somewhat detached from from the real me - if by nothing else, then by several decades. Still, that pic of your's doesn't do me sufficient justice...