Gunsmithing Freebore vs “Jump”

[krw]

Technically what is difference Freebore and Jump? I had PTG make me a reamer. Sent them 3 dummy cartridges. 260 Rem with SMK 123 lded 2.800” oal. I specified .010” jump. That is the way they ground it. The FB is marked .087”. Maybe a better question would have been what is freebore? Thanks!

 

[natdscott]

Freebore is a chamber dimension, and "jump" is what a bullet has to do to cross the freebore.

Freebore is a (usually) caliber-diameter section of the chamber AHEAD of the neck that basically just removes the rifling. The rifling is then cut into a TAPERED ramp to full height (the "leade"), the angle of which is usually between 1.25 and 2.0 degrees for match rifles, the most common being 1.5 degrees.

More freebore = more jump and less pressure, all else being equal. Less freebore = more pieces of rifle lying all over the range if you don't know what you are doing.

All they did with your reamer is measure the distance to the end of the bearing surface on all three rounds, average it, and then spec the reamer to cut the leade to be complete at ~0.010" ahead of that point. The NUMBER of the freebore on your reamer is just what they came up with from your rounds.

Whether that dimension does what you intend or not, they don't have to care. They just grind steel.

I'm gonna be honest with you here...if you don't understand what freebore is, I question if you are ready to start specifying what you want in a reamer.

-Nate

 

[spife7980]

To add onto the above:

Now here is a clip from the saami specs, a saami chamber is 2.18-2.0615 for .1185 freebore, making your reamer .0315 short of saami. If youre going to be buying factory ammo make sure it chambers easily before you go firing off rounds jammed to high heaven, I dont shoot factory 260 so I cant say how hot they are loaded or not. But less space for gas to blow by before the bullet engraves int he lands means that more gas is trapped behind the bullet increasing pressures.

Also note, PTG uses a different lingo for their reamers, notice that what we call the freebore they call the leade and what they call the throat we would call the leade.

Read this http://bisonballistics.com/articles/an-introduction-to-rifle-chambers but to sum it up, know what you are trying to describe as someone else may have a different understanding and you need to be aware of these differences in terminology

 

[krw]

I might not have known the true definition of Freebore, But I knew exactly how far I wanted a 123 SMK to jump loaded 2.800”. Thank You both for your replys. Both were informative.