Freebore… how do you measure it?

[Old Navy]

Agreed. All I really care about is the measurement that the video I linked above shows because that allows me to get significantly under half moa and that's good enough for me. ... 0.2 - 0.4" @ 100yd.

The discussion above is just a bunch of people telling everybody else they got it wrong. Lol.

I'm sure I got it wrong too but I don't care. What I'm measuring works for me and gets me the accuracy I need.

Grandman you can make an educated guess using your BTO measurements when buy a new barrel if you know the freebore of either one. Just bought a new BRA barrel from the Urban Rifleman. Brad said the freebore is. .104" I have a 6 BR Critorion barrel that I didn't know the freebore on. The BTO of the bullets I shoot measure within .003" in both barrels.

 

[Old Navy]

No, it isn't, not even close. On a reamer drawing there is a dimension indicated from the forward edge of the 45 degree chamfer at the end of the neck to the point where the throat angle begins. Typically, on a custom reamer the freebore is less than a thousandth over nominal bullet shank diameter, but this can vary. For instance the first design of the Wheeler 6BRA calls for a FB diameter of .244.

I tried seating a bullet in the case backwards and got measurement of .165 on a .104 freebore. You just told me why. That .244 or .245 makes a difference.

 

[ckaberna]

On a .22-250 factory Remington I owned years ago, I made up a plug to check the freebore. The large diameter on this plug is bullet diameter. I squeezed the neck down to fit the smaller diameter.
View attachment 1282094

I made this exact device and used it with a “modified case” and the hornady comparator.

 

[kvd]

The drawing is not correct in that the freebore extends forward to the point where the leade angle (in this case 1 degree 30 minutes) begins. The drawing incorrectly shows a gap.

I would be somewhat concerned about that 0.232 dimension. Isn't that a little small if going into a 0.236/0.237, 6mm bore? - or am I reading that incorrectly. Interesting thread. Clearly we are not all on the same sheet of music - so to speak - regarding free bore and how to measure it if the reamer or reamer drawing are not available.

 

[243winxb]

My 243 win has no freebore.

 

[Edd]

I would be somewhat concerned about that 0.232 dimension. Isn't that a little small if going into a 0.236/0.237, 6mm bore? - or am I reading that incorrectly. Interesting thread. Clearly we are not all on the same sheet of music - so to speak - regarding free bore and how to measure it if the reamer or reamer drawing are not available.

That is a reamer drawing. That .232 dimension is the dimension of the reamer. The reamer has a 1.5° taper. if you are reaming a .236 diameter hole, you extend the reamer length to a diameter smaller than the bore so you can get a clean cut. All reamers are made like that.

 

[Edd]

My 243 win has no freebore.

Yes it does. It has about a 1/16" of freebore.

 

[284winner]

None much

 

[gunsandgunsmithing]

This reamer's freebore is .070. The drawing is not correct in that the freebore extends forward to the point where the leade angle (in this case 1 degree 30 minutes) begins. The drawing incorrectly shows a gap.
View attachment 1282121

Working backward, from the end of the freebore...
We have
1.603 ..minus
.070 ...freebore gives us
1.533. So far, so good.

The .083 dimension is the freebore plus the length of the 45 degree bevel at the end of the chamber. In this case, using simple math, the difference is
.013.

1.603 - .083= 1.520
1.603 - .070= 1.533
1.533 - .013 = 1.520

I'm guessing you're referring to the .013 dimension but its stated by being expressed as .083...by adding it to the .070 fb length.


So, where's the gap?

 

[kvd]

The reamer has a 1.5° taper. if you are reaming a .236 diameter hole, you extend the reamer length to a diameter smaller than the bore so you can get a clean cut. All reamers are made like that.

Thank you for the information. I was hoping that dimension had a logical explanation. Now I can read reamer drawings with more understanding.

 

[243winxb]

Yes it does. It has about a 1/16" of freebore.

THROAT​

The tapered portion of the bore of a barrel, immediately ahead of the chamber which is sized to provide clearance for the bullet of the loaded cartridge. (SAAMI Definition)

SAAMI & I , think its a throat?? 243 Winchester Cartridge.

 

[BoydAllen]

Working backward, from the end of the freebore...
We have
1.603 ..minus
.070 ...freebore gives us
1.533. So far, so good.

The .083 dimension is the freebore plus the length of the 45 degree bevel at the end of the chamber. In this case, using simple math, the difference is
.013.

1.603 - .083= 1.520
1.603 - .070= 1.533
1.533 - .013 = 1.520

I'm guessing you're referring to the .013 dimension but its stated by being expressed as .083...by adding it to the .070 fb length.


So, where's the gap?

On the drawing the line that represents the leade angle stops well short of the dimension line for the front of the freebore. That is the gap I was referring to. It is not something that shows up in the given dimensions but if one took the drawing too literally, it might be confusing.

 

[JFrank]

On the drawing the line that represents the leade angle stops well short of the dimension line for the front of the freebore. That is the gap I was referring to. It is not something that shows up in the given dimensions but if one took the drawing too literally, it might be confusing.

Boyd
If i can't take a mechanical drawing literally than what can I rely on ?
The leade is the freebore y or n

 

[BoydAllen]

Boyd
If i can't take a mechanical drawing literally than what can I rely on ?
The leade is the freebore y or n

NO.

 

[M-61]

So, in a smooth bore it would be from the case mouth all the way to the end of the barrel.

Yes.

 

[DaveTooley]

What Boyd said

 

[AckleymanII]

There is usually .012-.014 that comes off the 45 degree angle of the case mouth, this amount varies also, add this to the print numbers

 

[JFrank]

NO.
View attachment 1282253

Excellent' now I know.
Thx Boyd thx Dave

 

[pirate ammo]

Yes.

You mean I got something right, I was thinking that I was the biggest dumb a-- here.

 

[Edd]

THROAT​

The tapered portion of the bore of a barrel, immediately ahead of the chamber which is sized to provide clearance for the bullet of the loaded cartridge. (SAAMI Definition)

SAAMI & I , think its a throat?? 243 Winchester Cartridge.

Did you miss post #35?