Calculating Freebore

[DirtySteve]

Thanks to @Jud96 for the interesting question. I’m trying to follow this but a few pictures with some labels or a link to an article with some drawings would make it a lot easier.

Some one lost me when they said, "Simple" and "Trig" in the same sentence...lol..

 

[Alex Wheeler]

Thank you! It seems most of it is trial and error. I would like to be able to figure out the dimensions I need before ever making a dummy round or ordering a reamer, but it seems it may not always be exact.

You can also scratch that line on the ogive and boat tail then measure between them to get the bearing surface. If you know the neck length of the case you can work with those numbers to get close as well. But this wont be exact. When JGS gets it right it will be within .001" and they often do. But every once in a while they come up short, I dont know if its a certain ogive shape or something else that causes that.

 

[Edd]

Looking at the case drawing: The distance from the base of the case to the neck shoulder junction is 1.6512". The length of the bullets bearing surface is .408".

If you put the rear of the bullet bearing surface at the neck shoulder junction, the distance from the base of the case front of the bullet bearing surface will be 2.0592", 1.6512" + .408" = 2.0592".

Looking at the chamber drawing: The distance from the bolt face to the front of the chamber transition angle is 1.9418".

If you put the front of the bearing surface at the front of the freebore, the freebore will be .1174", 2.0592" - 1.9418 = .1174".

The throat angle on this chamber is 1.5° and the freebore diameter is .0005" larger than the bullet. With a 1.5° angle, a .0001" increase in diameter will move the bullet contact point forward about .002". That means the bullet contact point on this throat will be about .010" beyond the front of the freebore.

So if you want the bullet to be touching the lands, you need to decrease the .1174" freebore by .010".

 

[Dusty Stevens]

Im going to guess you need a freebore of .140-.150 and whatever diameter works with your barrel. Thats just from past experience though.

 

[DaveTooley]

Now that some are completely confused with the math here's a simple way. Scribe a line on the ogive as Alex has explained. Measure from the base of the cartridge to that line. Give the reamer manufacturer that number and let them fill in the blanks. You get close to your desired COAL without errors stacking up.

 

[VA_XTC_Shooter]

Looking at the case drawing: The distance from the base of the case to the neck shoulder junction is 1.6512". The length of the bullets bearing surface is .408".

If you put the rear of the bullet bearing surface at the neck shoulder junction, the distance from the base of the case front of the bullet bearing surface will be 2.0592", 1.6512" + .408" = 2.0592".

Looking at the chamber drawing: The distance from the bolt face to the front of the chamber transition angle is 1.9418".

If you put the front of the bearing surface at the front of the freebore, the freebore will be .1174", 2.0592" - 1.9418 = .1174".

The throat angle on this chamber is 1.5° and the freebore diameter is .0005" larger than the bullet. With a 1.5° angle, a .0001" increase in diameter will move the bullet contact point forward about .002". That means the bullet contact point on this throat will be about .010" beyond the front of the freebore.

So if you want the bullet to be touching the lands, you need to decrease the .1174" freebore by .010".

Question, when I look at the print, the distance from bolt face to front of chamber transition angle (front end of "freebore") is 2.1247", then the 1.5* angle begins.

The 1.9418" is from bolt face to the angle leading into the throat. Wouldn't the freebore based on the 2.1247" measurement instead? Maybe I am thinking of it wrong, but I think of freebore as the space between case mouth and the beginning of the lands, which allows for seating depth variations.

 

[Edd]

Question, when I look at the print, the distance from bolt face to front of chamber transition angle (front end of "freebore") is 2.1247", then the 1.5* angle begins.

The 1.9418" is from bolt face to the angle leading into the throat. Wouldn't the freebore based on the 2.1247" measurement instead? Maybe I am thinking of it wrong, but I think of freebore as the space between case mouth and the beginning of the lands, which allows for seating depth variations.

The amount of freebore depicted on this drawing is .1829".

 

[VA_XTC_Shooter]

The amount of freebore depicted on this drawing is .1829".

I understand the freebore measurement from the reamer print, what I am confused about is how your above calculations with the reamer dimensions and bearing surface get the desired freebore for that specific bullet.

 

[Edd]

I understand the freebore measurement from the reamer print, what I am confused about is how your above calculations with the reamer dimensions and bearing surface get the desired freebore for that specific bullet.

 

[VA_XTC_Shooter]

Yes, exactly. So, I see 0.0692" (2.1247-1.6475-0.408) from front bearing surface to end of chamber freebore. Add your 0.010" estimate for lands contact, you will be 0.0792" from the lands if the bullet is seated with the rear bearing surface at the shoulder/neck junction, not 0.1174"

 

[Edd]

Yes, exactly. So, I see 0.0692" (2.1247-1.6475-0.408) from front bearing surface to end of chamber freebore. Add your 0.010" estimate for lands contact, you will be 0.0792" from the lands if the bullet is seated with the rear bearing surface at the shoulder/neck junction, not 0.1174"

You are calculating bullet jump on an existing chamber. The OP was asking how to determine the freebore needed for a custom chamber.

 

[VA_XTC_Shooter]

Isn't jump and freebore related? If you have long freebore, you have long jump.

 

[Edd]

Isn't jump and freebore related?

Yes they are.

If you have long freebore, you have long jump.

Maybe, maybe not. It depends on the bullet dimensions and placement.

 

[Ned Ludd]

For general purposes, these types of calculations (estimates) do not necessarily need to be rocket science. Given where most shooters would prefer to seat the bullet boattail/bearing surface junction in a case neck (i.e. somewhere from slightly above the neck/shoulder junction to perhaps a little more than halfway out the neck), you will generally have approximately half the neck length to play with. Having approximately half the neck length to work with is a fairly forgiving window. With that window in mind, errors in measurement/estimate in the range of +/- .010" are trivial, whereas errors on the scale of +/- .050" would probably not be trivial. Aiming for seating the desired bullet with its boattail/bearing surface junction somewhere in the middle of that range in the neck (i.e. above the neck/shoulder to slightly over halfway out the neck) when making a freebore estimate, and then simply accepting that there will be errors in the actual measurement and/or estimate, there will usually still be enough safety margin that the bullet will be seated with its boattail/bearing surface junction somewhere within the desired region of the case neck at the desired seating depth. When making such measurements/estimates, do not forget to take into account the likely optimal seating depth for the bullet of choice. For example, if you are interested in optimizing freebore length for a bullet that you know likes to be jumped .050" or more, that needs to be included into the estimate. I mention this largely because the measurement/estimate approaches we might commonly apply to this question are often done with the bullet seated at "touching" the lands, which is easy for most of us to feel when using the tools we have.

Other ways to refine the estimate might include comparison of CBTO at "touching" with a chamber of known freebore (same cartridge/caliber) using the same bullet and a Hornady OAL gauge (Stoney Point tool), or simply comparing BTO and/or bearing surface length measurements between two different bullets one of which is a known reference in terms of loading/seating for that particular chamber. Both of these approaches really require either a reference chamber or a loaded round with a different bullet for comparison, but they can usually get you close enough for practical purposes. As mentioned above, the preferred seating depth of the bullet needs to be included in the estimate if it likes a really long jump, or likes to be seated well into the lands. If you absolutely require a measurement that is more accurate than "somewhere between slightly above the neck/shoulder junction to just over halfway out the neck", buying your own reamer with defined dimensions is probably a good idea. In the grand scheme of things, the cost of a custom reamer is not that great, given that you probably spent at least ten to twenty times more on the rifle itself, and the fact that a reamer can be used for repeated barrel chamberings.

 

[VA_XTC_Shooter]

Ned, thanks for the summary. That is why I am trying to make sure I understand the concept and how to measure correctly. I currently have a 6BR with 0.094" freebore and it shoots 70 to 105VLDs amazingly well. I went to order a new reamer so I am ready to rebarrel when the time comes and most "off the shelf" have 0.105" freebore. This just seemed too much to me and along with my own experience with the 0.094".

I don't understand why the 6BR seems to have garned a longer freebore when from what I can tell, it is unnecessary. Now, is the longer freebore a detriment? I am not sure, I believe it is serving others quite well.

 

[Frankm]

....and effectively your calculated ‘free bore’ changes as you get 200, 400, 600, etc rounds down the barrel. I ve heard a couple reamer manufacturers say you can increase your free bore by shooting it, by using a throater, etc, but you can’t shorten it once you ve taken it out!!