AusVarmint

HI
Can you guys please explain free bore in chambers and on reamers.

Was talking to a G-smith and his 30 BR reamer is a "No freebore" reamer.

What does no freebore mean?
Does the chamber finish in a sharp angle and the rifling start straight away or is there still a blending of chamber and rifling?
Is free bore and leade the same thing??

I am trying to work out a beginning seating depth and have got 2 projectiles,
one a FBHP and one a HPBT that are similar weight.
I am trying to seat everything a minimum of 50 thou above the neck shoulder join area to avoid the doughnut formed when expanding the necks up with a tapered expander from 6mm to .30 cal. I am doing this as I dont have anyway of removing the doughnut inside the necks and I dont want the pressure problems associated with projectiles in the doughnut area.

THanks

later
p

I'm no expert on this but I'll provide my understand and welcome others to comment.

Freebore and leade are not the same. To my understanding both form the throat of a rifle chamber. Generally, where freeboring is applied, it is the cylindrical shape section immediately ahead of the chamber and is generally of groove diameter. Weatherby is famous for their use of freebore. They used to have approx 3/4" freebore but reduced that to 3/8" in the 80's. Freeboring reduces pressure but decreases accuracy potential as the bullet leaves the cartridge neck and is not supported when it slams into the rifling. If the freebore section is larger than groove diameter then the bullet is unsupported when it leaves the case neck.

In some rifle throats the freebore or unrifled section ahead of the neck can also be shaped like a truncated cone. This is generally reserved for dangerous game calibres and some lever guns. Examples include 458 WM and 375W. It has the dual effect of reducing pressure and allowing dirty ammo to chamber easily. As you can imagine accuracy potential is not great but its not needed either.

The leade is the gentle tapered transition section from the end of the freebore to the rifling. In the case of no freebore it can start from the end of the neck. The leade angle can vary from 0.5 to perhaps up to 3-4 degrees. Leades will change on use and sharper leades will settle down to 1-2 degrees pretty quickly.

Most factory rifle chamberings have little to no freebore. This generally provides the best accuracy. The throat is the most important part of the rifle chamber as it imparts the direction to the bullet. Large freebore, sloppy throats, cock-eyed throating is not conducive to good accuracy. Tread carefully in this area to avoid disappointment.

I'm not sure what your exact application is but it sounds like your setting your self up to have restricted loading options by doing what you are suggesting. Personally I'd look to see how to eliminate the doughnut. It would provide more loading flexibility.


Joe

HI Ogre , slip over to the Saubier site and have a look at the 17 VR reamer print and look at it in conjunction with JFE's post. After looking at the drawings it should be as clear as mud.

Looks like you already have!!!!!!!!! I should have read more before posting


Cheers

Ogre,

Freebore is basically the extension of the lead (throat) past the 'standard' length of the chamber, the main purpose is to allow the seating of longer length projies. Bloody hard to explain but here goes.........

Zero freebore means there is no throat diameter cut past the lead.

The chamber usually finishes at a 45 degree angle coming down from the neck dia to about a half thou' larger than the caliber.

The lead is measured from the largest dia of the 45 angle forwards...or the end of the case mouth.

Freebore is measured fom the smaller part of the 45 degree angle (0.3085 dia) forwards into the rifling.

The 'throat' usually ends at a 1degree 30minute angle into the rifling at whever the chosen point is...for freebore.

Mate don't send yourself stir crazy with all this reamer talk, if your smith is worth his salt...he will know what he is giving you is OK

As for your doughnut problem.......UMMM I dunno where your at with all this but.....

The 'doughnut' is formed during the neck expanding process, it has to occur as the brass from the shoulder is thicker than the brass in the neck. It will be gone before you fire your first shot so is no problem.

It's a piece of cake.....but use plenty of lube on the expander and go slowly as you can 'bust' a few necks going up to 30 fom 6mm in 1 pass

Once you have your necks expanded, set your neck turning cutter to just take the 'doughnut' off. Then reset to get down to your chosen neck thickness...if a 'tightneck' chamber is used.

It sounds as though your gunsmith has the 'Robinette' 30BR reamer as it is a zero freebore job, it's designed to work with projies made on about the 1"jacket length of 110 to 125grains. If so all will be fine

I am intending to do a bit of a 30BR build story on the forum soon....just gotta find some time

KY