Math for Computing Freebore Order

A

abintx

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I'm looking for a number I can pass on to a gunsmith when chambering a new barrel.

I know the case neck length (.203"), and bullet length from base to ogive (.4270").

The dimension from the case head to ogive just touching the the lands is 1.904". I seat the bullet .010" longer at 1.914".

When I do that I get a nice square mark on the bullet just ahead of the case neck. However, the bullet is being pushed back into the case below the neck/shoulder junction.

Can someone tell me how I compute how much freebore to order to prevent the bullet from being pushed below the neck/shoulder junction ???

Your assistance would be appreciated.
 
There isn't any special math to compute freebore. Freebore is the straight section ahead of the chamber neck. That straight section is defined by diameter and length.

For example on a 6mm PPC with a 0.262" neck. The neck diameter of the chamber would be 0.262". The normal freebore would be about 0.2435" diameter since the normal 6mm bullet is 0.2433" diameter.

The infamous "Boyer #3 drawing from Kiff ,the freebore is 0.2433 diameter by 0.06" long. The "Boyer III from Kiff by Mike Marcelli, the freebore is 0.2435" diameter by 0.055" long.

I shoot a 40 degree shoulder 6PPC that has a freebore of 0.2435" diameter by 0.085" long. In my chamber, a 66 Fowler, 68 BartsDDNS, 68 Watson, etc., the straight part of those flat base bullets seat about 0.110" into the case neck.

Seat a bullet to where you want it to be and take that to your gunsmith. Unless he has a throating reamer he will be constrained by the reamer he has or has ground.
 
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case lengths

If you're trying to relate freebore to the overall seating measurment remember the case length will come into play. A chamber with a 1.525 length and .070 freebore is not going to have the same measurement to the lands as one with a 1.500 length and the same freebore.
 
Jerry:

JerrySharrett said:
There isn't any special math to compute freebore. Freebore is the straight section ahead of the chamber neck. That straight section is defined by diameter and length.

For example on a 6mm PPC with a 0.262" neck. The neck diameter of the chamber would be 0.262". The normal freebore would be about 0.2435" diameter since the normal 6mm bullet is 0.2433" diameter.

The infamous "Boyer #3 drawing from Kiff ,the freebore is 0.2433 diameter by 0.06" long. The "Boyer III from Kiff by Mike Marcelli, the freebore is 0.2435" diameter by 0.055" long.

I shoot a 40 degree shoulder 6PPC that has a freebore of 0.2435" diameter by 0.085" long. In my chamber, a 66 Fowler, 68 BartsDDNS, 68 Watson, etc., the straight part of those flat base bullets seat about 0.110" into the case neck.

Seat a bullet to where you want it to be and take that to your gunsmith. Unless he has a throating reamer he will be constrained by the reamer he has or has ground.
Click to expand...

1. How were the lengths of 0.06", 0.055" and 0.085" determined ??? (I have no problem with the diameter.)

2. Where do you seat your bullets in relation to the lands ??? Just touching, into by 0.010", into by 0.020", etc. ??? I would imagine this comes into play.

Just trying to wrap my head around how the freebore length is computed. (Based on how the bullet is seated and how the square mark on the bullet is achieved.)
 
abintx said:
1. How were the lengths of 0.06", 0.055" and 0.085" determined ??? (I have no problem with the diameter.)

2. Where do you seat your bullets in relation to the lands ??? Just touching, into by 0.010", into by 0.020", etc. ??? I would imagine this comes into play.

Just trying to wrap my head around how the freebore length is computed. (Based on how the bullet is seated and how the square mark on the bullet is achieved.)
Click to expand...
1 (above)-these dimensions were determined by what bullet (flat base/boat tail, nose ogive, and jacket length) you plan to shoot, and, where you want the bullet to sit in the cartridge case. You can't make a freebore to shoot them all. But, you can pick a bullet and seat it to where you want it to be then send that assembled (DUMMY) round to a reamer grinder and he will grind you a reamer to make it work.

2 (above)-I seat the bullets, based on testing to where it shoots the best in benchrest situation. In the benchrest situation you will only be moving the bullet just a few thousants in or out till you find that sweet spot. In the hunting situation you have to, first, setup a dummy round to fit the rifle magazine and so on.
 
Everyone calls freebore something different so the best advise has already been given. Take a round to your gunsmith just like you want if possible.
Some people call the straight area from where the neck area of the chamber ends to the start of the throat the freebore or the leade. Personaly what I call freebore is the distance from the end of the case neck to where the bullet makes contact with the rifling. This is a combination of the safety factor from the end of the case to the end of the neck area of the chamber. About 0.010. The 45 degree transition area from 0.262 neck to 0.2435 of the leade. About 0.020. And depending on the throat angle maybe another 10 or 15 thousands before the bullet touches rifling. So I would have 0.040 or more with no leade at all. I would add whatever leade to this as my total freebore. Jst 2cnt wrth Kenny
 
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