Determining Freebore

[abintx]

What factors are used in determining the freebore for your rifle's chamber ???

How do you decide how much freebore to order for a new barrel ???

Any chart that can be used for this task ???

Thanks for your reply.

 

[jackie schmidt]

Art

Freebore is dictated by what bullet you wish to use, and how far into the neck you want it to be when just touching the lands. It seems that having the base of the bullet between .150 and .190 down in is about right in a 6PPC.
The standard for years was the .060 freebore, in conjunction with a 1.525 long 6PPC chamber. This was the JGS 1045 as a good example. This grind worked particularilly well with Bullets with relative straight shanks and low number o-gives, such as Fowlers and Watsons.
I know shooters who have increased the freebore clear out to .090, wanting more capacity. Personally, I think that is going the wrong way. One reason the 6PPC shoots so well is that magic relation between the affective chamber volume and the bullet cross section. When I went to the 00 boat tail bullets, I went to a "zero" freebore to get the capacity back where it should be.
I have a feeling that Gene Beggs might be on to something with his 6mm Beggs, which has even a smaller capacity. Only time will tell. As soon as Gene gets me a sizing die, I will have one up and running for my Rail Gun.
One note, when I went to the 'zero" freebore, velocity went up with my standard load by an average 40 fps, showing that I got the peak of the pressure curve back toward the chamber, where I think the best accracy is.
I know a lot of this sounds like voodoo, and nonsence, but these are the little things that shooters are always playing with within the basic "PPC" design in order to get a little better agging capability........jackie

 

[Charles E]

Just one thing to add to Jackie's post -- the half-cone angle is also a factor.

My reamer has a .090 freebore length, but has a 2-degree half cone angle (4 degrees included). With that setup, it puts a bullet like a Fowler 66 at just about the same spot as a .050 freebore length with the more common 1.5 degree half cone angle. In a pinch, I can even shoot the 62-grain Fowlers, which are made on a .750 jacket or a bullet made on a .790 jacket.

There are a range of bullets this combination will work with, from the 7-ogive Fowlers to the 8.5-ogive BIBs, and even the 9-ogive Knights, which shoot quite well in my rifle. I don't believe any of these are "double radius" bullets. Still, I guess I wouldn't recommend it (the .090 freebore with a 2-degree half cone angle); it is an old-fashion design, as are most of the bullets I mentioned.

None of this is meant to contradict what Jackie says, but rather augment his idea that just what bullet(s) you pick is a factor in freebore length.

 

[abintx]

Jackie and Charles:

Are there books or other literature that I can read that will cover the points you brought up. I'd like to expand my knowledge and satisfy my curiosity on the finer points of chamber design, tooling, internal ballistics and bullet relationship to these factors. I'm not an engineer but would like to understand some of the minutia and some of the variables you consider when you build a benchrest rifle.

I have a subscription to Precision Shooting, and own copies of The Ultimate in Rifle Accuracy, Extreme Rifle Accuracy, and The Benchrest Shooting Primer. I don't want to build a rifle myself but want to be able to converse intelligently when talking to someone that's a recognized builder/gunsmith. Your recommendations would be appreciated.