Where is Freebore measured from

[NJRaised]

I understand what Freebore is. But where is is measured from? The front the neck?

Example… smith gives me a Freebore measurement of .085” Freebore for. 223 rem. Is that tacked onto the 1.750” case length?

 

[Supersubes]

No, measured from the end of the chamber. There’s clearance and a 45 degree transition forward of where your case neck ends.

 

[Rio Precision Gunworks]

In the chamber, there will be a spot where the neck tapers at 45deg. It tapers from the neck diameter/clearance down to the clearance in the freebore.

That area will remain the same diameter until it starts a lead angle ( 1deg is common) that tapers down to the bore size (bore size will be smaller than bullet diameter).

The freebore is the area where the 45deg taper stops and the lead angle starts.


Here is a 6gt Saami print. I've highlighted the 45deg taper in yellow, the 1deg lead angle in green. And the freebore in red. You'll notice the freebore area has a diameter of .2436 squared in red. That means the freebore will have .0006" clearance for the .243 bullet in the freebore.

 

[Rio Precision Gunworks]

Now, you might get some confusion if someone just takes a measurement or makes a cast of the chamber.

As technically, on a loaded round.....where the neck stops and up until the ogive touches the rifling would be what you could call the "effective freebore." As there will be no rifling touching the bullet in that area (in a perfect world, nothing at all touching the bullet, but every round has runout, and its usually larger than the clearance in freebore, so there is actually steel touching the bullet there).

 

[longrange772]

In the chamber, there will be a spot where the neck tapers at 45deg. It tapers from the neck diameter/clearance down to the clearance in the freebore.

That area will remain the same diameter until it starts a lead angle ( 1deg is common) that tapers down to the bore size (bore size will be smaller than bullet diameter).

The freebore is the area where the 45deg taper stops and the lead angle starts.


Here is a 6gt Saami print. I've highlighted the 45deg taper in yellow, the 1deg lead angle in green. And the freebore in red. You'll notice the freebore area has a diameter of .2436 squared in red. That means the freebore will have .0006" clearance for the .243 bullet in the freebore.


View attachment 8367738

@Rio Precision Gunworks This is a very helpful diagram. So our trim to length for our brass is ensuring we don't push up against the section in yellow, correct? And, the yellow section is where the carbon ring grows?

 

[Baron23]

@Rio Precision Gunworks This is a very helpful diagram. So our trim to length for our brass is ensuring we don't push up against the section in yellow, correct? And, the yellow section is where the carbon ring grows?

Ah, I’m pretty sure you want to trim right up to the step down 45 angle bit) and no further.

 

[Supersubes]

@Rio Precision Gunworks This is a very helpful diagram. So our trim to length for our brass is ensuring we don't push up against the section in yellow, correct? And, the yellow section is where the carbon ring grows?

Ah, I’m pretty sure you want to trim right up to the step down 45 angle bit) and no further.

Most loading manuals set trim length ~.020 short of the chamber length. You can work closer if you want.