Discrepancy between cartridge measurement and reamer prints

[VA_XTC_Shooter]

I feel like this may be a dumb question and I am just having a bad day, but here goes.. lol

I understand every reamer has little tweaks here and there, but I am noticing all the 6BR reamer prints I have seen have smaller sizes than the cartridges for the base to shoulder neck junction dimension.

base to shoulder neck junction of the 6BR cartridge is 1.2378". However, the reamers are shorter at 1.2346" to 1.236".
Why is this? I would think it this could lead to chambering brass sized to CIP minimum headspace, no? Yes, I know headspace is off the datum, but how else do you move the should neck junction toward the base 1-2 thousandths to fit in the chamber?

Speaking of headspace, The JGS print is vastly different than PTG.
JGS lists 1.670"-1.1770" for min/max headspace
PTG lists 1.1453"-1.495" for min/max headspace

This is because they measure to a different datum point (JGS at 0.3500" diameter and PTG at 0.375" diameter). How come there is not consistency there?

 

[KRP]

The neck diameter of the chamber is larger than the neck diameter of the cartridge, trigonometry accounts for the difference in length.

If you look at a SAAMI cartridge drawing you'll see a x.xxxx - .007 dimension at the shoulder diameter corresponding to the headspace datum of the chamber at a length somewhere around the middle of the chamber headspace dimension.

SAAMI has no recommended dimensions for the 6BR, you can measure where you want. CIP spec is a different story.

Cartridge & Chamber Drawings – SAAMI

 

[AM_ARixon]

To your first question/point. There are a few things that can make the comparison of the shoulder neck junction between brass and chamber drawings seem odd. First, accuracy of measuring tool and accurately determining the intersection of lines, something that is not that easy unless using shop based QC equipment. Measuring this on a case by had and eye is tough and seeing your measurements are only a couple thou out, I'd say you are well within error, dependent on measuring equipment...

Second, the reamer chamber drawing can be a little confusing... I've drawn up a number of reamers from prints supplied from reamer manufacturers and many of the dims do not intersect correctly if you hit the exact numbers on the print. Some can be close, I've seen a few +/-.005 apart. I can't explain this lol. Then once manufacturing that reamer the tolerances in the diameters and angles will influence those exact intersection points too. I'm not sure which measurements these manufacturers hold as most important but we hit shoulder angle and headspace distance then go from there. So the reamer drawing may not be a true representation of the exact chamber when you dig into the details of them.

Third, the saving grace, there is a very generous radius at that intersection which pretty much takes account for everything I just said above and should give you tons of room for your brass to fit into no problem at all! So yes, you may be seeing that distance from base to intersection of shoulder and neck but I wouldn't worry about it.

To KRP's point above, the 6br is only a CIP based cartridge so always refer to their standard drawings for the case and chamber. Lapua brass and all other brass manufacturers should be making brass to the CIP standard as that is the only standard out there for it, and what the original lapua was made to. People will modify and change reamer prints for their specific situation, either knowing or not totally knowing how it can impact performance so just because a print says it is a 6br, doesn't mean it will always work for your case.

 

[VA_XTC_Shooter]

I did not measure my brass. I am comparing CIP cartridge numbers to reamer numbers.

 

[KRP]

I did not measure my brass. I am comparing CIP cartridge numbers to reamer numbers.

What is it you're looking to do? More information on your part would give everyone else a better chance to provide useable input.

 

[AM_ARixon]

The difference between your reamer print to cip drawing print is due to the individual who created the drawing. If the person who created your reamer print made slight body diameter or neck diameter changes then it will change the intersection point of the neck and shoulder. It really depends on how that person created that reamer print. I always look at shoulder angle and length from base to the datum line and then design the reamer off of those dimensions. Designing like this the only difference in that intersection point comes from the diameter difference of the neck. But as you state above, the two measurements are within a few thou of each other, this is nothing to worry about due to the large radius provided at that intersection point.

 

[VA_XTC_Shooter]

What is it you're looking to do? More information on your part would give everyone else a better chance to provide useable input.

I am just trying to learn.

I ordered a reamer for my 6BR. As I was combing through reamer prints and CIP dimensions, I noticed what I consider discrepancies.

I understand my 2nd comment about different headspace measurements as that is arbitray depending on where from datum you measure.

I still don't understand why a reamer would be "shallower" than the brass dimension. The CIP spec for base to shoulder/neck junction is 1.2378" (this is NOT my measurement, but straight from CIP and other sources identifying 6BR case measurements). All of the reamer prints indicate they cut shallower than this depth by about 1-2 thousanths.

I am guessing this is an effort to "tighten up" the chamber?

 

[VA_XTC_Shooter]

For comparison
CIP for 6BR Norma (in mm, so need to convert to in)

Attached is the JGS reamer print