Making a datum line gauge

[Gregg C]

Got a somewhat perplexing dilemma in that I need to know the datum line to base dimension of my big Wby cases, and have not had any success finding a gauge. Sooo, I got .420 gauge thingy from Hornady, thinking tht might be close enough to the shoulder , but I think it registers too close to the neck to really tell me what I want to know. The case drawing shows the diameter of the shoulder is .560 , which I think is the datum line, from what I understand . Spent cases from the rifle measure approximately .5625in. I thought I might drill the .420 gauge out to a larger size. Say .500. Or maybe closer, .5468. Comments invited, encouraged even. Thanks in advance,
Gregg

 

[Edd]

The datum line diameter is .4728" on a 338-378. which cartridge do you have?

 

[Gregg C]

338-378. So is that measurement between the two radii?
Gregg

 

[Edd]

338-378. So is that measurement between the two radii?
Gregg

Yes

 

[Gregg C]

Yes

Ah. Thanks for setting things rite.
Gregg

 

[Gregg C]

The datum line diameter is .4728" on a 338-378. which cartridge do you have?

One more question, and then MAYBE I'll understand. This measurement, the datum line, to the base, is this where you check shoulder bump back, fired case to sized case comparison? Thanks.
Gregg

 

[CaptnC]

I just got a Hornady head space in the mail today...as I understand it...

Yes, that is what/where you're measuring...

 

[Trickymissfit]

goto a bearing shop (like Dixie, Motion, or Bearing Inc.) and buy a 12mm inner race for a needle bearing. Just make sure it's longer than the neck length. The are ground with pretty sharp corners and are parallel at the ends. Rest is clearance , so doesn't matter much. Then you just measure over the bushing and the base of the case. Been doing this for years.
If you just have to use a Hornaday gauge, then you'll need to turn one out of steel or aluminum. The inner race is more accurate.
gary

 

[Gregg C]

goto a bearing shop (like Dixie, Motion, or Bearing Inc.) and buy a 12mm inner race for a needle bearing. Just make sure it's longer than the neck length. The are ground with pretty sharp corners and are parallel at the ends. Rest is clearance , so doesn't matter much. Then you just measure over the bushing and the base of the case. Been doing this for years.
If you just have to use a Hornaday gauge, then you'll need to turn one out of steel or aluminum. The inner race is more accurate.
gary

Thanks for that, sounds like a good idea. I had a drill bit that measures .480, so I drilled out the .420 bushing I bought today. Seems to work ok. We will see. Thanks again,, folks.
Gregg

 

[bigedp51]

Don't over think the Hornady gauge and the SAAMI datum point.

You are just measuring a fired case and a resized case for shoulder bump. The datum point is to make sure a headspace gauge measures chamber headspace at one point of the chamber.

And you are just "comparing" the two case lengths after sizing for proper shoulder bump.

Below a Colt 5.56 Field gauge at 1.437

Below the same gauge in a adjusted Hornady gauge. But it is still just a comparison reading because of brass spring back after sizing and you average a few cases to set the die.

Below a "fired" Lake City case from my AR15 in the Hornady gauge. And then the die is adjusted for .003 shoulder bump.

And your shoulder bump is your head clearance when the cartridge is chambered. And the distance the rear of the case has to stretch to meet the bolt face. And .001 to .002 head clearance is well within the brass elastic limits and not stretch and thin when fired.

 

[Gregg C]

Don't over think the Hornady gauge and the SAAMI datum point.

You are just measuring a fired case and a resized case for shoulder bump. The datum point is to make sure a headspace gauge measures chamber headspace at one point of the chamber.

And you are just "comparing" the two case lengths after sizing for proper shoulder bump.

Below a Colt 5.56 Field gauge at 1.437

Below the same gauge in a adjusted Hornady gauge. But it is still just a comparison reading because of brass spring back after sizing and you average a few cases to set the die.

Below a "fired" Lake City case from my AR15 in the Hornady gauge. And then the die is adjusted for .003 shoulder bump.

And your shoulder bump is your head clearance when the cartridge is chambered. And the distance the rear of the case has to stretch to meet the bolt face. And .001 to .002 head clearance is well within the brass elastic limits and not stretch and thin when fired.

Thanks for your time. I think I got it. Averaging because the brass measurement won't repeat (spring back?) exactly from case to case. Use an average to set the size die to push the shoulder back .002-.003in. And don't worry about the actual head space, as that is taken up(within reason) by the fire formed case. Am I missing any thing ? Thanks again.
Gregg

 

[bigedp51]

Thanks for your time. I think I got it. Averaging because the brass measurement won't repeat (spring back?) exactly from case to case. Use an average to set the size die to push the shoulder back .002-.003in. And don't worry about the actual head space, as that is taken up(within reason) by the fire formed case. Am I missing any thing ? Thanks again.
Gregg

Shoulder bump thumb rule
Bolt action shoulder bump .001 to .002
Semi-autos shoulder bump .003 to .006
Also on a semi-auto the resized diameter should be .003 to .005 smaller in diameter than its fired diameter after sizing. This insures the case will "spring back" from the chamber walls and extract reliably. Most semi-autos and pump actions do not have the same primary extraction that a bolt action has on bolt lift. Meaning the bolt lift is what breaks the case free from the chamber walls and secondary extraction is pulling the bolt straight back.