Measuring Free Bore in a Barrel

[Terry]

I did not want to hijack the other thread.

How do you measure free bore in a barrel?

 

[acloco]

PM sent.......

 

[Terry]

Let me state the question in a different way. A guy hands you a barrel and ask is this a .135 free bore or a .160 free bore? How do you measure it?

 

[DaveTooley]

Without more information about the chamber it's almost impossible to accurately determine a number. You could set it up in a lathe and determine the end of the neck and the end of the FB but that's just one number as it relates to seating depth.

 

[okie]

cast chamber and throat??

 

[Ned Ludd]

If you have a chamber of known freebore (same cartridge/caliber), you can use something like a Stoney Point tool, measure cartridge-base-to-ogive at "touching", then do the same with the chamber of unknown freebore. The difference between the two will give you a rough idea of how much freebore the 2nd chamber has. As Dave pointed out, this is a rough estimate only as there will always be some error in the measurement itself, and the chamber of known freebore may not be exactly what you think it should be (according to the reamer print), especially if it has several hundred or more rounds fired through it. Nonetheless, this approach may provide a "working number" that is sufficient for the reloading process; i.e. to point you in the correct direction as to where the boattail/bearing surface junction of a certain length bullet would be positioned in the neck at a given seating depth relationship to the lands.

 

[skeeljc]

Isn't freebore is relative to a specific bullet?

 

[Keith Glasscock]

Some things I’d like to try when I have time:

1) Use pin gauges, and see how far they do/don’t go. You should also be able to define the freebore diameter that way.
2) Set it up in a lathe and run an indicator in. There should be a parallel section, followed by a taper. I can imagine that the taper will show up.

Just some thoughts...

 

[DaveTooley]

Isn't freebore is relative to a specific bullet?

No it isn't. By design the freebore and lead angle( the throat ) are hard numbers. For SAAMI spec cartridges they are industry wide standards.
Seating depth varies with bullet design.

 

[Terry]

cast chamber and throat??

and then?

 

[okie]

and then?

try to measure with calipers?? sure plenty error here...

 

[skeeljc]

No it isn't. By design the freebore and lead angle( the throat ) are hard numbers. For SAAMI spec cartridges they are industry wide standards.
Seating depth varies with bullet design.

O.K. you are correct. Freebore is a hard number. Just read it from the reamer print.

 

[Terry]

 

[Terry]

Pictures help me understand.

 

[Terry]

Is this picture right?

 

[skeeljc]

Looks right to me.

 

[lpreddick]

Seat a flat base bullet backwards with minimal neck tension. Chamber it slowly so the bullet is seated as it's base touches the lands. If you measure the length of bullet from lip of case, this is freebore PLUS neck portion of chamber not occupied by the cases neck PLUS the small angle cut in chamber from chamber to bore. Even if no freebore, you will measure the later two of the above distances a bullet must be seated out before touching the lands.

 

[Alex Wheeler]

Accurately would be hard due to different groove diameters unless you can take a cast and put it under an optical comparator. I like Mr Tooleys idea of setting it up in the lathe and using the dial indicator and dro to get close.

 

[Terry]

Thanks to all!

 

[LRPV]

If you’re trying to compare two different reamers or determine if a given reamer is “correct” when comparing two chambers, you’re up against a hard spot. Freebore diameter and overall chamber length (case length) used by one and not the other can really skew results. I have had a .135 dasher reamer and a .115 measure basically the same seating depth. Neither were “wrong” just different.