Calculating Free Bore on 6.5 WSM?

[Edd]

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.

I think I can explain where the numbers came from.

The bullet diameter is .264" but it is difficult to identify the exact spot where that diameter ends with mechanical measuring tools. It is easier to locate the spot where the diameter is .262".

The .0477" is subtracted from the measurement because that is the distance from a diameter of .2645" to .262" at a 1.5° angle.

The method has a fallacy. If one wanted to use this method, you would need to know the angle of the bullet nose in addition to the throat angle and you would need to use .264" instead of .2645" in the calculation.

 

[J E Custom]

If a person looks at the SAMMI chamber dimension he will see many dimensions relating to requirements for his bullet diameter and cartridge.

To answer all of these questions you must first understand why dimensions are what they are and the reason for that dimension.

There are many ways to establish what you have first so you don't mis-measure or calculate the wrong dimension and make a mistake with the modifications.

Bottom line is that the chamber dimensions are decided on by some very smart people with lots of experience and should not be altered unless you have years of experience and a very good understanding of what the function of each dimension is.

There is so much misinformation available things can go wrong quick. My recommendation it to have someone with years of experience, and that understands the reason for, and function of those dimension and features "first" to evaluate what you have and what you desire/want and come to a consensus. That will achieve what you want without problems.

This advice is for everyone Including my self (I always talk to the experts in there field to get as many pieces of advice as i can before I start swimming in uncharted waters) and will hopefully prevent me or someone else from ruining a perfectly good rifle.

J E CUSTOM

 

[L.Sherm]

This is why I just send in a dummy round.

 

[Mram10us]

Everything is math. Call me crazy, but I don't see it as difficult. Years of experience shouldn't be needed to make some trigonometry equations and find a simple value. Another easy way is drill a piece of ... well anything with a hole that's .260ish. Stick it over the bullet like a comparator and find your value that way. We've already established for every .002" of bore different, the fb measurement changed .0038".

if you don't want the extra work, call JGS or whoever cause they'll most likely have a reamer spec for that bullet

 

[David Emerson]

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?

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?

An easy way is to seat the intended bullet to the oal length you want. Set your caliper to .263 and make a ring on the ogive of the bullet and then measure to the mouth of the case. Some people need to overcomplicate things. Keep in mind your 6.5 Winchester short will move the lands forward fairly quickly so I would subtract .040 from your measurement.

 

[L.Sherm]

I've never had my 6.5x300wsm move .040.
Theres alot of variables that make throats move.

 

[Edd]

An easy way is to seat the intended bullet to the oal length you want. Set your caliper to .263 and make a ring on the ogive of the bullet and then measure to the mouth of the case. Some people need to overcomplicate things. Keep in mind your 6.5 Winchester short will move the lands forward fairly quickly so I would subtract .040 from your measurement.

The freebore measurement doesn't start at the case mouth. There is typically about a .025" gap between the case mouth and the start of the freebore.

 

[David Emerson]

I've never had my 6.5x300wsm move .040.
Theres alot of variables that make throats move.

shoot it enough rounds and get back to me on that

 

[L.Sherm]

I have I'm on my second one.

 

[David Emerson]

I have I'm on my second one.

The main reason I said to go .040 shorter is as the throat moves forward he will not run out of magazine box.

 

[L.Sherm]

Spec your reamer at .180-.200 freebore and run atleast a 3.1 mag box and you wont have to worry about it up to the 140's

 

[L.Sherm]

It is certainly no worse than a 28 Nosler or any other cartridges that will run a 140 at or above 3200FPS.
Heat, long strings, cleaning practices etc.
have an effect. It's not a cartridge to sit down a bang away with or just plan on rebarreling

 

[BoatTail]

I think I'm going with a 0.200 Freebore which will allow me the option to shoot Hornady 143gr ELD-X, Berger 140 gr Elite Hunter and Nosler 142gr Accubond Long Range bullets. The Nosler bullets have a very long Boat Tail that limits how far it can stick out of the case. Using the measurement method I shared before the ABLR comes out with a FB of 0.202. If I didn't want this option and was going with just the Hornady ELD-X I could go with a FB of 0.252 to 0.262. However I want to see which of the three bullets shoot best out of this rifle and then I will go with that one. I wonder though if I should be adding 0.020 for bullet jump. Thoughts?

 

[L.Sherm]

I wouldn't add anything because you dont know exactly were the bullet is gonna shoot the best every rifle is different.
You can always throat longer but you cant put it back.