6mmBR Freebore Calculations (This shouldn't be so hard)

[Boatschool02]

Hello,
First post.
Huge fan of the caliber specific pages. Such a wealth of knowledge. Thanks.
--
I've commissioned my first 6mmBR rifle to be built upon a Borden BRM action.
In the course of my homework, I've read nearly every freebore related post on this site spanning more than eight years. A plot of the results across brands, bullet weights, bearing surfaces, and barrel diameters turns out to be less than helpful.
--
What I don't understand is this:
1. Just how differently can reamer manufacturers' "versions" of freebore calculations actually be?
-- If this is tangible, why can't we quantify it?
2. Why aren't people, especially among such a detail-oriented crowd, talking about bearing surface to ogive measurements instead of weight? (Weight being so misleading regarding ideal freebore, in this case.)

Any/all input is appreciated. While I understand that a "calculator-like" app may not be appropriate, and is likely unnecessary as reamer manufacturers don't charge to spec dummy rounds, I do feel that we can do better when discussing optimal freebore considerations WRT specific bullets.

If nothing else, an accepted "rough" methodology might help reduce redundant questions and enable readers to experiment with different bullets in a more calculated manner.

Thanks,
Luke

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Calculations based on Berger Quick Reference Sheet:

In hopes of sparking some interest (and in case my PDF attachment doesn't work) the first column represents four extreme examples of what I'll term "ogive protrusion" with the Flat Back or Bearing Surface heel 0.042" ahead of the NSJ. 0.042" leaves 0.100" of usable progression before encroaching on Sierra Tech Support's 80% of one caliber in the neck (+/- 0.180") recommendations. (Of interesting note, this is not possible with either the Berger Target or Hunting 95 grain VLD but IS POSSIBLE with the 87.)

The second column represents the associated, calculated freebore assuming .028" (.014 + 0.014) is removed from the calculation for case length and a 45 degree slope from neck to throat. Yes, this ignores any "jam" and any math related to throating angles. This is largely due to my election to measure forward from the bolt face vice backwards from the lands. If these assumptions can be removed or improved, please chime in.

Bullet //OP// **FB**
Berger 68FB Target: //.122"// **0.094"**
Berger 88FB Varmint: //.173"// **0.145"**
Berger 95 VLD Hunter: //.078"// **0.050"**
Berger 105 VLD Target: //.123"// **0.095"**

While it might be easy to dismiss the first two bullet calculations based on norms or convention, this method arrives within 0.010" of the forum's accepted advice on the both VLD bullets. If for example, I had selected 0.050" ahead of the NSJ, the 105 VLD Target freebore math would be 0.001" away from the frequently endorsed 0.104" norm.

Why does this method clash with the accepted norms (extensive interviews and posts by champions, smiths, and reamer manufacturers) for "lighter" Flat Back bullets, where the math is even easier?

EX: ((Bearing Surface - 0.280") (0.042 ahead of NSJ in 0.322 neck) + (Base to Ogive - Bearing Surface) - neck length adjustment (0.014) - lead adjustment (0.014) = Freebore

Even if my math is missing a critical element, why wouldn't the delta apply to all bullets working from the bolt face forward? Said differently, if the Ogive shape (secant vs tangent vs hybrid), throating angle, and barrel diameter hold the missing clues, what is the point of the manufacturer specified ogive measurements? (Notice how traditional shaped bullets yield a larger distance from Ogive back to the bearing surface.)

 

[timeout]

Way over my head friend. Looks to me like you should be answering questions, not asking. No offense meant. I simply load a dummy cartridge and send it to the reamer mfg. For most reamers I tend to just get a short throat and then use a unithroater to set the freebore where I want it. I know some do not like unithroating, so to each his own.

 

[expiper]

OP,,,,I grew up playing with a cap pistol,,,sounds like you grew up playing with a computer,,,,most shooters have learned thru trial and error what is long enuff or short enuff to work with certain bullets,,,I hope you enjoy your new rifle,,,OBTW,,,what freebore did you decide on for your chamber,,,or just leave it to the gunsmith,,,,Roger

 

[timeout]

OP,,,,I grew up playing with a cap pistol,,,sounds like you grew up playing with a computer,,,,most shooters have learned thru trial and error what is long enuff or short enuff to work with certain bullets,,,I hope you enjoy your new rifle,,,OBTW,,,what freebore did you decide on for your chamber,,,or just leave it to the gunsmith,,,,Roger

10-4 on the cap pistol. Me too!

 

[Boatschool02]

I wanted to try 95 VLDs and 95 SMKs for 300 yards and in informal target shooting.
The Bullet Guidance section suggests 0.035-0.055" for the 95 VLD.
Smith recommended 0.055" and that is what I went with.
Based on knowledge gained here, I knew I could always have it throated longer if I couldn't get the 95s to work. I enjoy the challenge.
--
This sprang out of a trip to Cabelas. Believe it or not, they had an amazing price on the 95 VLDs and some of the 87 VMAX that seem to be very popular on the forum. After looking at the 0.100"+ bearing surface difference between the bullets, I started to question the freebore guidance/norms based primarily on 60-80, 80-100, and 105+ weight classes.
--
After getting some more feedback from the experts, I plan to expand my bearing surface and base to ogive listing to include Sierra and Hornady.

The VLDs seem to make sense. I just don't understand where the math goes awry with flat backs. Either I'm missing something significant, which is entirely possible, or FB shooters are seating behind the NSJ unknowingly.

If my research comes in handy for others, great.

 

[bdale]

You did well with the .055, I have a 6BR that I built just to shoot 90 grain class bullets and after contacting Dave Kiff went with a .050 and most 90 and 95 grain bullets are just above the neck shoulder junction with the exception of the Berger 95 VLD which has a shorter bearing surface so it seats out a little further. With that said I chambered a 14 twist barrel with the same reamer to shoot lighter bullets 55 to 70 grain and the .050 freebore is a little much, not enough bullet in the case for me, have since bought a zero freebore reamer which should work well for those bullets

 

[SenderoMan]

Interesting topic to say the least!
I just went thru the same thing and my gunsmith recommended a .050 freebore for my 14 twist barrel to keep the base of the bullet above the shoulder junction.

bdale, I'll be curious to see how mine turns out, since you have tried the same combo

Please share your research as you progress with this project.

Thanks!

 

[Boatschool02]

Bdale,
Thanks for the input.
--
Maybe I'm not explaining myself very well.
IMHO, weight and bearing surface don't tell the whole story. In fact I'm suggesting they provide, at best, a non-linear relationship to ideal freebore dimensions.

I'm offering that we should be referencing the sum of the boat tail and bearing surface (bearing surface only for FBs) in relation to the neck length (presumably even more important on improved cartridges) AND the distance from the end of the bearing surface to the ogive.

Let's take the popular Berger 80 grain flat back varmint bullet as an example:
-Lapua 6mmBR case has a neck of +/- .322"
-The Berger 80 FB has a bearing surface of 0.372"
-- If you seat it @ the NSJ, you have 0.050" bearing surface exposed.
--- From there, you have another 0.094" out to the ogive, for a total "ogive protrusion" of 0.144"
- Subtract +/- 0.028" for case length and 45 deg transition and you've got 0.116" to the ogive "in the throat."
** This is at the NSJ and only grows as you advance the bullet.
- If we go with my 0.042" above NSJ #, then you've got .158" to the ogive in the throat.

How does this square with the "book answer" of a 0.060-0.080" freebore recommendation based on weight?
If the answer is an additional distance, currently missing from my model, required to transition to the .236/.237 bore at 1.5 degrees, I'd say: "Great. Makes sense. But why does that ruin all the previous math that seems to work for VLDs?"

On the opposite extreme, a Forum Administrator suggests that a 95VLD has only 0.135" in the neck with a 0.080" Freebore and 0.010" jam. Running the same numbers, that means 0.124" bearing surface outside the case, plus another 0.072" to the ogive for a total of 0.196. Subtract 0.028" and another 0.010 for jam and you have 0.158" to the lands. In other words, in their gun, with their reamer, with their bullet, a 0.080" freebore requires another 0.078" "missing" variable in order for the ogive to reach the lands.

If this 0.078" variable, that seems to be missing from my model, holds true across all bullets, that could/would make sense juxtaposed with the 80 grain Berger from above. Take the 0.158" we ended up with, subtract 0.078 and you've got 0.080" freebore which is right inline with conventional wisdom.

Again, I'd say great. This seems to make sense, except it means that anyone wanting to shoot the 95VLD with 0.200" in the neck with 0.010" jam would need a 0.015" freebore. (Entirely possible, but notably different than the prevailing recommended range.)

Attempting to apply the same math to the ubiquitous 0.104" freebore for 105 VLDs, yields 0.193" of the available 0.326" bearing surface in the neck, with ogive at the lands. Put differently, the base (heel?) of the bearing surface would be 0.133" above the NSJ and the base of the boat tail would be 0.083" behind the NSJ.
For those attempting to squeeze out the last bit of velocity, these numbers seem pretty reasonable.

So, that leaves a couple candidate conclusions:
1. Light, short flat back shooters are unknowingly seating behind the NSJ. (Unlikely)
2. I should have bought a 0.015" or even ZERO Freebore for the 95VLD contrary to common knowledge.
3. People sending in dummy rounds for reamer specifications are in fact, seating their bullets much closer to minimum bearing surface in the neck vice the 0.030-0.050" ahead of NSJ numbers that get pushed around.
4. There is an additional +/- 0.078" distance missing from my model.

So far, my money is on #s 2-4 being closest to true.

I know that I could have just bought a 0.104" freebore for the 105s or sent in a dummy round for the 95s, but that wouldn't get me any closer to buying cartons of 100 bullets with a reasonable expectation of them working in my rifle. One look at the drastic bullet to bullet differences in boat tail, bearing surface, and base to ogive lengths should make anyone feel suspicious about taking or giving a freebore recommendation based on weight.

I am continually impressed with the fanatical attention to detail applied by world class shooters on this forum. Given the cost of reamer, resize reamer, custom dies, and chambering (+/- $950), I'd like to think I'm not the only one who'd like to be able to make accurate predictions, on their own, from home.

Appreciate any/all feedback.
Thanks,
Luke

 

[expiper]

boatman,,,,you are worried about the size of the drops of sweat that are on the horses back that is pulling the wagon,,,,just settle on what bullet your bbl will shoot well and realize that there is no universal dim that solves all possible mathmatical equasions and theoretical bullet dims....just listen to the folks on here who have shot in world class event and set world records ,,and you will be off to a good start...you cant learn this from home,,you gotta go shoot,,,Roger

 

[dkhunt14]

I believe there are too many variables to figure it out like this. One lot of bullets can vary .030 or more in bearing length. A .236 barrel can be big or small, same as a .237. Then the diameter of the bullets can vary by .002. These all change where the bullet sits with the same reamer used. Matt

 

[WayneShaw]

A 6BR has a long neck. It is very forgiving. For the most part, you can live with two freebore lengths. First, a short length for up to a 80-90 grain, then long for the 105-107 class. I would go .040" for short, .090" for long. The throat grows. You usually hit pressure before running out of powder capacity.