Freebore… how do you measure it?

[Medic505]

Every freshly reamed barrel of mine gets a casting.

Why? And, I'm not being a smartass. What information do you gather by doing this that is different from what you'd get with a comparator?

 

[Fuj]

Why? And, I'm not being a smartass. What information do you gather by doing this that is different from what you'd get with a comparator?

It's a hard copy of what your reamer actually did. You'll also see how long
the neck actually is and it's actual width for turning purposes. Besides the
the throat and lead, I like to see how tight the throat is.

 

[Medic505]

It's a hard copy of what your reamer actually did. You'll also see how long
the neck actually is and it's actual width for turning purposes. Besides the
the throat and lead, I like to see how tight the throat is.

I guess all that would depend on what temperature your casting is and how well you could accurately measure. So many variables.

 

[SweaterDood]

I guess all that would depend on what temperature your casting is and how well you could accurately measure. So many variables.

Unless you have access to (and the operation knowledge of) laser scanning devices, chamber castings are going to be as accurate as it gets. The shrink ratio of the casting won’t be enough to matter for the purpose of measuring freebore or neck turning diameter.

I do agree on the importance of the operator’s ability to use a measuring tool, though.

 

[M-61]

It's a decent tool however you can easily take 5 measurements using exactly the same bullet procedure etc., and achieve 5 different sizes. So I pick the 2 that are the closest and go from there. It's a good idea but it has its limitations. I don't remember getting the exact measurement twice ever. Design and user are two huge factors. For me it's 'close' enough.

 

[hogpatrol]

The issue with the above is how hard the rod is pushed up against the base of the bullet can vary the reading. I put the bullet in the case and keep the rod backed off and locked. Turn the barrel to muzzle down and let the bullet fall in to the lead angle. Unlock the rod lock and let it fall on to the bullet base, Lock the rod, remove it, remove the bullet and check. I can do this ten times and get a measurement within a couple thousandths.

Edit: I use the Wheeler method on a new barrel with the bullets I plan on using, then use the Hornady for comparison.

 

[Fuj]

Unless you have access to (and the operation knowledge of) laser scanning devices, chamber castings are going to be as accurate as it gets. The shrink ratio of the casting won’t be enough to matter for the purpose of measuring freebore or neck turning diameter.

I do agree on the importance of the operator’s ability to use a measuring tool, though.

Yes Sir !! I use Roto Metals 158/190 F. chamber cast. There really
is no shrinkage to worry about. Sometimes though, the filling in
of imperfections makes for a harder removal.

 

[darkangel_r2]

It's a decent tool however you can easily take 5 measurements using exactly the same bullet procedure etc., and achieve 5 different sizes. So I pick the 2 that are the closest and go from there. It's a good idea but it has its limitations. I don't remember getting the exact measurement twice ever. Design and user are two huge factors. For me it's 'close' enough.

1. You need to make sure the modified case is securely tightened all the way down.
2. You need to maintain slight forward pressure on both the case and the rod that pushes the bullet forward when the screw is tightened.
3. Use the flat anvil attachment for the bottom of the brass. This help center the comparator measurement consistently.

Doing these 3 things always has kept the measurements within a thou or two for three measurements with the same bullet.

If you know the leade angle of your chamber you can calculate a good estimation of your freebore. The base to ogive measurement this tool gives minus the length of the brass will give you an approximate measurement of freebore + leade length. If you know the leade angle and the outer neck diameter (round up to the nearest whole .001" for spring back) of a fired piece of brass can use some trig to remove the leade length. Then you are left with freebore.

It looks like this.

B2O=cartridge base to ogive measurement
BL=Brass length
Y=fired neck outer diameter(rounded up)-bore diameter (land diameter not groove ex. for 308 its .30" typically) DIVIDED BY TWO

So the equation is.

Freebore~=(B2O-BL)-(Y/tan(leade angle))

Casting a barrel will give a more accurate measurement but that's a lot of hassle on an assembled gun.
You may also be able to get an approximation off a rigid bore scope either from the muzzle (be careful) or from the chamber with the tightest bore guide the scope can fit through. Mark on the bore scope rod right when you see rifling, light change from leade angle change, and brass neck step, on the top or bottom of the mirror (use the same side for each mark. Never tried this but may work for a rough number. If you try this might be a good idea to do it a few times and make a determination off of an average or something.

 

[JFrank]

The issue with the above is how hard the rod is pushed up against the base of the bullet can vary the reading. I put the bullet in the case and keep the rod backed off and locked. Turn the barrel to muzzle down and let the bullet fall in to the lead angle. Unlock the rod lock and let it fall on to the bullet base, Lock the rod, remove it, remove the bullet and check. I can do this ten times and get a measurement within a couple thousandths.

Edit: I use the Wheeler method on a new barrel with the bullets I plan on using, then use the Hornady for comparison.

An excellent master rifle smith told me essentially the same thing a few years ago.

 

[rwj]

Unless you have access to (and the operation knowledge of) laser scanning devices, chamber castings are going to be as accurate as it gets.

Are there laser scanning devices for internal surfaces?

 

[SweaterDood]

Are there laser scanning devices for internal surfaces?

Yes, but they are typically for larger cavities. The typical scanner should be able to accurately measure depth of a relatively shallow hole. I am not an expert on their use, but I believe the depth of a chamber should be within the scope from my limited experience. I would run a dowel into the barrel to where it would contact a loaded round. It should plot a pretty darn accurate chamber profile mesh.

 

[Fred Bohl]

Safer than metal casts you might try http://www.reprorubber.com. This will produce a detailed and exact replica. For the best accurate measurements they are usually done with an optical comparator or CMM. With care adequate measurements can be made with hand tools bearing in mind that this cast is flexible.

 

[tsyarnall]

Safer than metal casts you might try http://www.reprorubber.com. This will produce a detailed and exact replica. For the best accurate measurements they are usually done with an optical comparator or CMM. With care adequate measurements can be made with hand tools bearing in mind that this cast is flexible.

Looks to be a good product.

 

[JMayo]

R to L
That's a case in the chamber then free bore then lands groves ... Correct ?

 

[urbanrifleman]

This is the best method.

 

[tsyarnall]

That is A method…….

 

[fatelvis]

I would assume when you are opening the action using that method, you might want to give the bullet some help with a cleaning rod down the bore while extracting? If not, I would think you might stick the bullet in the lands, or pull it out of the neck a little?

 

[Medic505]

That's the "I'm only going to stick in the tip" method.

 

[urbanrifleman]

I would assume when you are opening the action using that method, you might want to give the bullet some help with a cleaning rod down the bore while extracting? If not, I would think you might stick the bullet in the lands, or pull it out of the neck a little?

You can pinch the slot together to give yourself the perfect amount of grip. The bullet should move, but with difficulty.

Add a tiny bit of sizing wax to the tip. Then it won't stick. Sometimes it will but try again.

There will be a number that it lands on over and over then it's done

 

[urbanrifleman]

That is A method…….

This is the BEST method for finding "touch". And it works even if the barrel is out of the action. When I set freebores with a throating reamer I can push the case in with my finger and get perfectly accurate repeating measurements.