How to Precisely Measure Chamber dimensions after reaming

[Cassidy]

Webster, by neck turning the case neck which some turn to within .0001" of their target/desired neck wall thickness, I can adjust the case neck thickness to get a .004" (or whatever value the shooter selects) gap (.002 each side) between the case neck and chamber neck. Instead of relying on the reamer spec sheet which may or may not cite a tolerance for the reamer neck OD dimension (which means the reamer neck OD might vary within the cited tolerance, though I could measure it), and also recognizing there might be a difference in the chamber neck ID depending on how the gunsmith reamed it, if I have the actual measured chamber neck ID (using for example a 3 point tube mic, which I don't have), I might be able to more precisely determine and neck turn to the target/desired case neck thickness to get the gap I want (which might also be based on group size and/or presence of sine waves on the case necks after shooting the cases a few times). This is what I'm thinking.

 

[Cassidy]

Webster, once it's chambered, if I can accurately measure neck ID, I can adjust the case neck thickness I turn to obtain the gap I desire between the case neck and the chamber neck (.004 meaning .002 on each side, or whatever I decide is most accurate) to achieve the smallest group sizes. This way I'm not relying on the reamer spec sheet which has a range of tolerances, or variances that might be due to differences in gunsmith reaming methods.

 

[Cassidy]

Like Tim suggested above, I think maybe I'll try a series of plug gauges (each to .0001) carefully inserted into the chamber neck to see which ones fit into the neck and which ones don't fit (go, no go) to get what might be a fairly precise measure of the actual chamber neck diameter. If it works this will give me a numeric measurement.

 

[BuffaloBill]

To measure the chamber’s neck diameter, I turned a length of brass rod, sneaking up on a diameter that would lightly friction-fit into the neck. Then measured that with a micrometer. I used wet/dry paper for the last few tenths.

 

[Piglet]

Why wouldn't a chamber cast and .0001 micrometer be precise enough for neck diameter?

 

[Jonnyofalltrades]

Why wouldn't a chamber cast and .0001 micrometer be precise enough for neck diameter?

I've never tried to measure a cerrosafe casting that precisely, (I'm usually after fairly coarse measurements with cerrosafe) but I can't imagine getting anything repeatable in the tenths range.
It's just not that precise.

 

[jackieschmidt]

You can get extremely accurate readings on small holes with this. This is a Starrett small hole gage I use on my .330 neck 30BR. Chambers.

It does take a little skill, but with a little practice, anybody can master it’s us.

 

[Lucky Shooter]

This question for Jackie and other highly skilled machinists.

How small resolution can you get with the Starrett Small Hole
Gage ?

Thanks.

A. Weldy

 

[DShortt]

Does the rifle shoot better or worse knowing all details down to a gnats ass?

Good Lord. Y'all make my left eye twitch sometimes.

 

[DaveTooley]

This question for Jackie and other highly skilled machinists.

How small resolution can you get with the Starrett Small Hole
Gage ?

Thanks.

A. Weldy

I reverse engineer chambers and other things occasionally. Never had a problem getting close enough using them.

 

[jackieschmidt]

This question for Jackie and other highly skilled machinists.

How small resolution can you get with the Starrett Small Hole
Gage ?

Thanks.

A. Weldy

Probably a couple of “tenths”. Using items such as this does require a certain amount of acquired skill in learning exactly how to use it.

But then, contrary to popular internet belief, all precision hand tools do still rely somewhat on operator skill.

 

[Lucky Shooter]

Thank you all for replies.

I got a number and realize I have some learning to do.

A. Weldy

 

[Cassidy]

At the top of this subforum there is a post (the first one cited) entitled, "130 page PDF of different chambering methods" which appears to be written by "Mike". On the bottom of page 22, the last paragraph states,

"I had a rifle in the shop recently that was assembled by a well known and well advertised gun builder (and expensive!!!). The rifle is a hunting rifle in the big 30 caliber. This rifle had a lot of expensive truing and re-machining accomplished by said gun builder. It was shooting 1-1/2 to 2 inch groups at 100 yards. After I took the recently installed new barrel off and took measurements, I found the chamber was 0.004" out of being concentric with the bore, the chamber was 0.006" larger at the chamber mouth than SAAMI standard and the receiver "truing" was accomplished incorrectly. I installed another new barrel, chambered it correctly and corrected the receiver machining error. I shot a three group with this rifle that was less than 1/4 inch at 100 yards after I corrected all the problems."

This may be an unusual example and is of a hunting rifle, but it underscores that different methods used by different gunsmiths may result in different machining outcomes which can effect group precision.

For my BR rifle, which now has slightly more than 1000 rounds through it, the Deltronic gauge pins I ordered came in and after applying light oil (0W-20), I carefully and gently tried to see if they would fit into (all the way) the chamber neck. The first gauge pin I tried (diameter accurate to 4 digits after the decimal; i.e.,. the last digit was a tenth of one thousandth) was the same diameter as the reamer printout for the neck, and it fit into the chamber neck. Next, I used a gauge pin that was .0001" larger in diameter, and it also fit into the chamber neck all the way. Third, I used a gauge pin that was .0002" larger in diameter, and it went about 2/3rd the way into the chamber case neck (no pushing). Based on these data, I think my gunsmith did a pretty good job. This seems to me to be a reasonable way to obtain a pretty close number for chamber neck diameter. Use of gauge pins in this manner will not, however, tell me if there is a concentricity problem with he bore.

I would like to know, though, recognizing I am not a gunsmith, and also recognizing that reamer neck tolerances cited on reamer prints may be +0.0005" or so, what do BR gunsmith's think is an acceptable tolerance if the measured chamber neck diameter is .000X" or .00XX" greater than cited on the reamer print. Please advise me, what is an acceptable X for BR shooting?

 

[DShortt]

At the top of this subforum there is a post (the first one cited) entitled, "130 page PDF of different chambering methods" which appears to be written by "Mike". On the bottom of page 22, the last paragraph states,

"I had a rifle in the shop recently that was assembled by a well known and well advertised gun builder (and expensive!!!). The rifle is a hunting rifle in the big 30 caliber. This rifle had a lot of expensive truing and re-machining accomplished by said gun builder. It was shooting 1-1/2 to 2 inch groups at 100 yards. After I took the recently installed new barrel off and took measurements, I found the chamber was 0.004" out of being concentric with the bore, the chamber was 0.006" larger at the chamber mouth than SAAMI standard and the receiver "truing" was accomplished incorrectly. I installed another new barrel, chambered it correctly and corrected the receiver machining error. I shot a three group with this rifle that was less than 1/4 inch at 100 yards after I corrected all the problems."

This may be an unusual example and is of a hunting rifle, but it underscores that different methods used by different gunsmiths may result in different machining outcomes which can effect group precision.

For my BR rifle, which now has slightly more than 1000 rounds through it, the Deltronic gauge pins I ordered came in and after applying light oil (0W-20), I carefully and gently tried to see if they would fit into (all the way) the chamber neck. The first gauge pin I tried (diameter accurate to 4 digits after the decimal; i.e.,. the last digit was a tenth of one thousandth) was the same diameter as the reamer printout for the neck, and it fit into the chamber neck. Next, I used a gauge pin that was .0001" larger in diameter, and it also fit into the chamber neck all the way. Third, I used a gauge pin that was .0002" larger in diameter, and it went about 2/3rd the way into the chamber case neck (no pushing). Based on these data, I think my gunsmith did a pretty good job. This seems to me to be a reasonable way to obtain a pretty close number for chamber neck diameter. Use of gauge pins in this manner will not, however, tell me if there is a concentricity problem with he bore.

I would like to know, though, recognizing I am not a gunsmith, and also recognizing that reamer neck tolerances cited on reamer prints may be +0.0005" or so, what do BR gunsmith's think is an acceptable tolerance if the measured chamber neck diameter is .000X" or .00XX" greater than cited on the reamer print. Please advise me, what is an acceptable X for BR shooting?

Do you want want the truth, or what sounds good?
Because the truth is, no one knows.
There are things that point toward consistency. And there are examples of deviations.
I'd suggest you experiment and see what works for YOU.

 

[Will Henry]

Does your reamer print show a straight neck or a tapered neck? .0005 to .0015 of taper is not uncommon. As far as the precise diameter of the neck is concerned, I think this is of much less importance than is concentricity. In fact, I KNOW it is of much less importance.
If the reamer cuts a throat which is .0005" over the groove diameter of the barrel, you can readily see .0002" misalignment with the naked eye (or in my case, with my bifocals). The thing is, you almost have to make a conscious effort to get a reamer to cut all that much over size.
I know one gunsmith who was packaging a reamer up to return it when I dropped in to visit. I asked him why and he said it was cutting over size. There was nothing wrong with the reamer. His tailstock was mis-aligned and he had not checked it. WH

 

[jackieschmidt]

Do you want want the truth, or what sounds good?
Because the truth is, no one knows.
There are things that point toward consistency. And there are examples of deviations.
I'd suggest you experiment and see what works for YOU.

^^^^^^^^^
Pretty good advice.

 

[Cassidy]

Will - the print sheet shows the same four digits after the decimal for the reamer case neck. Thanks for the advice re concentricity.

 

[DShortt]

Will - the print sheet shows the same four digits after the decimal for the reamer case neck. Thanks for the advice re concentricity.

You also mentioned that you have 1K rounds down this barrel. Whatever it is now, it's not identical to as-delivered condition.
That's simply fact.

 

[Cassidy]

DShortt - that's why I mentioned it. Even with about 1K through the barrel, the chamber neck diameter is still only about 0.00015" or so greater than the stated reamer print neck diameter which is why I said I think my gunsmith did a great job 1K rounds ago. I sure would like to know what BR gunsmiths on this site think a reasonable 0.000X or 0.00XX" reamed chamber neck diameter is beyond the stated reamer print neck diameter since you guys know the limits/tolerances of your equipment and methods.