Calculating Free Bore on 6.5 WSM?

BoatTail

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I need help figuring out how to calculate Freebore (FB) for a 6.5 WSM custom chamber reamer for a new rifle build I want to do.

Over the last few years I have had a number of custom chamber reamers made for various builds. In each case I either had a SAAMI reference chamber and drawing I could measure off of to make the FB adjustments I wanted or used the FB measurements for a similar cartridge I liked and trusted.

I'm now trying to determine the FB for a 6.5/300 WSM to allow the most case volume. I don't have a reference chamber to measure off of so need to make the calculation. The FB on the 6.5 PRC is a good reference (0.1879) but again wanted to make it optimum.

Here are my components:
  • Remington 700 Short Action with a 3.110 extended well magazine and extended ejection port
  • Winchester 270 Brass resized to 6.5/300 WSM
  • Hornady 143gr ELD-X and Nosler 142gr Accubond Long Range bullets
This is one of the methods I was given to calculate Free Bore:

For a cartridge in 264:

Tools and components:

  • Begin by cutting a notch out of the neck and shoulder of a donor case to be able to see where the bullet rides in the case at various COALs
  • Color the bullet nose of the bullets with a dark permanent marker
  • Subtract 0.002 from the cartridge bullet diameter (0.262)
  • Set your micro meter to 0.262 and mark/etch a circle around the bullet.

  • Set the bullet in the case so the base of the neck and top of the boat tail of the bullet are aligned.

  • Measure from the base of the case to the line marked earlier, just covering the line.

  • Take that measurement and subtract 0.0477 ( I assume that this has something to do with subtracting the difference from Lead and Free Bore and were FB starts)
  • Take that measurement and subtract the case length (2.1 for my case)
  • This then gives you the free bore
  • My added step to to check the COAL to make sure it fits with clearance in the 3.110 magazine I bought from PTG.


What I don't know is if I then add 0.02 to allow for bullet jump or not.

With the ELD-X and ABLR's this gives me a free bore around 0.252 for the custom reamer.

Using the measurements from the 2019 Berger Bullet Quick Reference Sheet I backed into the FB for a Berger 6.5 140 gr Elite Hunter and came up with 0.246 FB to ensure the top of the boat tail and bottom of the case neck met and thus use all of the neck for proper tension.

These FB dimensions seem excessive. Most of the drawings I have seen for the 6.5 WSM have a FB between 0.180 and 0.205 and as I said the 6.5 PRC FB is 0.1879.

How do you calculate the Free Bore for a new custom reamer?
 

WildRose

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Personally I'd just discuss it with whomever I chose to build the dies and cut the reamer.

Shouldn't really matter much because it's a wildcat as you just need a measurement determining the distance from the neck of the case mouth to ogive plus then the ride into the grooves.

The angle of that ride is important, if it is too sharp then pressure spikes very quickly.
 

BoatTail

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Making a mock up cartridge with bullet and the COAL i want and sending it into the Reamer maker as you suggested is the other method I was given. However I want to understand how they then calculate freebore so can figure it out for myself and then understand the variations if other bullets are used. In my case from Nosler, Hornady and Berger.
 

WildRose

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Making a mock up cartridge with bullet and the COAL i want and sending it into the Reamer maker as you suggested is the other method I was given. However I want to understand how they then calculate freebore so can figure it out for myself and then understand the variations if other bullets are used. In my case from Nosler, Hornady and Berger.
I would think you need to figure it based on the longerst bullet from base to ogive you intend to use.

there have been a lot of 6.5-.300wm's built so finding a drawing online shouldn't be hard.

Who are you having build the reamer and dies?
 

BoatTail

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I'm using PTG. I have several 6.5/300 WSM drawings with FB's of 0.100, 0.153, 0.188. I thought I had one with a 0.205 FB but that was not the case. With the largest of these being 0.188 and the 6.5 PRC FB being 0.1879, my 0.252 FB calculations seem way off. Help me understand why.
 

FEENIX

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Making a mock up cartridge with bullet and the COAL i want and sending it into the Reamer maker as you suggested is the other method I was given. However I want to understand how they then calculate freebore so can figure it out for myself and then understand the variations if other bullets are used. In my case from Nosler, Hornady and Berger.
As WR noted you need to work/consult closely with your gunsmith, reamer maker, and custom reloading dies. In my case, the reamer was built by Manson Reamers and John Whidden (he needs 3 fire-formed brass off the chamber) did my custom hydraulic forming die and custom reloading die set for my wildcat.

For what it is worth ...


 

L.Sherm

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I'm not understanding why subtract the .002 from .264.
My thinking is you engage the lands at .264 the dia. of the bullet.
Measure from .264 and see what you come up with.
 

BoatTail

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The person that gave this to me is prominent in the industry. Probably should talk to him and find out why but my thinking is that subtracting .002 from the caliber and marking that on the bullet, makes that Mark farther out towards the tip meaning that this is where it actually engages the lands. In other words the bullet and lands don't hit hitat the full diameter of the bullet instead it hits farther down on the diameter of the bullet.
 
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Danny1788

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Because a .264 barrel the “bore” is not actually .264” that is the depth of the groove. The bore is smaller so you will contact before .264”

I'm not understanding why subtract the .002 from .264.
My thinking is you engage the lands at .264 the dia. of the bullet.
Measure from .264 and see what you come up with.
Think bullet engraving to the groove the lands is the bore size.
 

Mram10us

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Is your goal to have the lower portion of the bullet body (not including the boat tail) just above the neck/shoulder junction of the case?
 

L.Sherm

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I understand the grove is .264, actually the bore is .256 on a 6.5 but on a reamer the throat area is always atleast .0005 bigger like .2645 so why wouldnt the throat stop were it transitions back to .264 seems to me that's were the bullet will engage at .264
 

Mram10us

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Saami specs don't give a measurement (have to find the difference between the 2 lower numbers on the chamber prints) for the distance from the .2645" throat to the .256". Here are some numbers to further cloud the conversation :)

The -.002" idea seems sound. The math gives you .0038" past the .2645" throat diameter

Did the math and it is .1623" from the .2645" to the bore diameter of .256"

Edit: I forgot they posted the numbers on the bottom of the chamber saami print. just find the difference between the bottom values and there is your freebore.
 
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L.Sherm

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I'm certainly no expert on this but this is how I understood it when I have talked with Grant and Jason at JGS.
I always just send in a dummy round and they send me back a print and have me confirm everything is o.k before they put it into production.
 

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