Myth Busters II – Bob Crone’s 6mm BRX – by Robert Whitley
by Robert Whitley
After Accurate Shooter published the July 31, 2010 Daily Bulletin feature on 6mm BRX Myth-Busting With Bob Crone, further research reveals that there’s more to tell about the BRX, its origins, and its original specifications.
6 BRX Myth-Busters Follow-Up Challenge
I wanted to obtain a print reflecting the true original dimensions of Bob Crone’s 6mm BRX reamer. I hoped to verify the head spacing of the reamer to be .100″ longer than a 6mm BR, and I also wanted to see how Bob’s original print compares to 6mm BRX match reamers currently considered to be the “standard in the industry”.
Recreating the Original 6mm BRX Reamer Print
Clymer Precision was contacted to obtain a print of Bob’s original reamer. Todd Wilms, the current owner of Clymer Precision (and a heck of a nice guy as well) went back in the Clymer archives and pulled the records of the dimensions of Bob Crone’s 6mm BRX reamer. Todd then entered them into Clymer’s current reamer drawing program and has generated a current drawing reflecting the original 6mm BRX reamer. The print (see below) confirms the head space of the 6mm BRX to be .100″ longer than a 6mm BR (i.e. 1.267″ min at a shoulder datum diameter of .350″).
Comments on Bob’s 1996 Clymer Reamer
In many ways, the print dimensions are very close to what is currently recognized as the “standard in the industry” for 6mm BRX reamers, however in other ways there have been some definite changes:
A. Body Diameter: A chamber body diameter of .470″ is still in wide use today with 6mm BRX reamers (that’s measured .200″ forward of the bolt face). The current Lapua 6mm BR brass has a web diameter very close to this diameter and one should be wary of trying to tighten this dimension. If anything, running a little looser (like the .4708″ dimension similar to that used with the 6mm Dasher) would not hurt the performance of the 6mm BRX cartridge a bit.
B. Shoulder Diameter: Bob’s original reamer had a shoulder diameter of .4609″ (the same as a 6mm BR Norma), however many current 6mm BRX reamers now use a .460″ shoulder diameter which helps avoid overworking the brass there if readily available commercial dies are used.
C. Max Chamber Length: Bob’s original print had a “max chamber length” of 1.570″ (the same as a 6mm BR Norma), however many of the current 6mm BRX reamers have shortened this up a little. This is because, when 6mm BR brass is fire-formed into 6mm BRX brass, typically the brass shortens. Indeed, this author’s 6mm BR brass shrinks in OAL from around 1.555″ to around 1.547″ when it is fire-formed to 6mm BRX brass. A max chamber length in the range of 1.560″ – 1.565″ is seen often with 6mm BRX reamers these days.
D. Neck Diameter: While Bob’s reamer has a .262″ neck, this has for the most part given way to either a .269″ (sometimes .268″) turned neck, or a .272″ (sometimes .271″) no-turn neck, with the neck diameter typically selected based on the actual dimensions of the brass to be used.
E. Throat Angle: Bob’s throat angle was one and a half degrees, which seems to be standard and in wide use today with the 6mm BRX.
F. Freebore: Bob’s freebore length was 0.00″ because his smith used a separate throater to set the throat length Bob wanted for specific bullets. These days, a built-in freebore in the range of .104″ – .125″ seems to be very common with 6mm BRX reamers. Freebore in that range is seen by many as a good choice for the readily available 105- to 108-grain 6mm bullets.
6mm BRX — Real World Specs
Here are measurements on Robert Whitley’s 6mm BRX brass, as fire-formed, full-length sized and ready to load in a no-turn chamber.
Base to shoulder = 1.170″
Base to neck/shoulder junction = 1.333″
Overall case length (fired 2x and re-sized 2x) = 1.547″
Shoulder diameter (resized) = .459″
Diameter .200″ forward of base (resized) = .470″
Neck diameter loaded = .269″
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Tags: 6BRX, 6mmbr, Bob Crone, Clymer, Dasher, Robert Whitley
It would be interesting to know what the base size of Robert’s brass was before sizing…don’t you think? One would assume that a knowledgeable designer would build in a little clearance between his chamber and a sized case. If he did, then his chamber is bigger than .470. If it is not, then his die isn’t doing any sizing at that point on the case….not good. Also, this is a twice fired and sized case. As brass work hardens, the amount of diameter reduction, at the base of a case, produced by a given die, will decrease, due to increasing spring back, which means that if you want a die to work at the end of a case’s life, you need more clearance built in than is actually needed for smooth functioning of a new case.
Boyd
Funny you should mention that as I addressed the issue you raised in the info submitted for publication, but it was edited out by the publisher (I assume to make the piece short enough to fit in the bulletin). Just remember, a bulletin item is a brief blurb, not a full story.
Robert Whitley
Boyd,
Here are Roberts comments on the body dimensions of the BRX:
Here’s some info for you on BRX brass (resized and ready to load).
Base to shoulder = 1.170″
Base to neck/shoulder junction = 1.333″
Overall case length (fired 2x and re-sized 2x) = 1.547″
Shoulder diameter (resized) = .459″
Diameter .200″ forward of base (resized) = .470″
Neck diameter loaded = .269″
Some comments:
The BRX reamers I have (both Kiff and JGS), per the prints, are .470″ in diameter at .200″ ahead of the base of the case, but honestly, the chambers with both reamers wind up maybe .001″ – .0015″ larger there for a few reasons:
a. The tolerance on the prints is + .0005″ and after having a variety of reamers measured in a comparator, it seems most reamer makers seem to stay in the plus side of the tolerances (probably for liability reasons); and
b. What I typically see is that reamers tend typically to cut a slightly larger than they actually measure (even the best of the best); and
c. Chamber polishing takes a little more.
The net result is I see fired brass at .200″ forward of the bolt face at .471″ – .4715″ in diameter, so a re-sized dimension of .470″ works.
For day to day resizing I use a 6mm BR Redding Type S FL sizing die with the lock ring set so the die sits up off the shell holder about .100″ from where it would sit for 6mm BR case resizing (i.e. the BRX has a .100″ longer head space than a 6mm BR). To hit the base, I use my 6mm Dasher Redding Type S FL sizing die with a .022″ thick cardboard spacer ring I made under the lock ring, and with the de-capping rod assembly and neck bushing removed. This Dasher die with the spacer seems to hit the base of the BRX case close enough to the web to bring it down for comfortable chambering and extraction, and it keeps the shoulder at around .4585″ – .459″ for re-sized brass (vs the fired brass that measures right around .460″ at the shoulder). When I used a .308 Win die to hit the base, while the base was fine, at the shoulder I was seeing .4565″ – .457″, which was more than I wanted to see. The Dasher die works great, because I have the lock ring set up for my work with the Dasher, and I just remove the de-capping rod and bushing, put the cardboard spacer under the lock ring and use it for the BRX base. A quick do-it-yourself cardboard spacer (anywhere around .022″ – .026″ works fine) can be made from the cardboard that a number of shooting components come in (the cardboard that holds sleeves of 1000 CCI or Federal primers, the cardboard from older Lapua brass boxes, the cardboard from Lapua bullet boxes, etc.).
Robert
My first BRX reamer was a total disaster. I had to trim 6BR brass .015″ before they would chamber in the BRX. I called the maker Dave Kiff, and he told me that was the original Crone design, that he had to get permission to copy the design. I will take a picture of the reamer print for display if anyone wants to see it. After this story I know [the reamer was not correct].