Measuring Finished Chamber Dimensions

[Jud96]

Please post if you figure out a solution. My OCD now wants to measure chamber!

I have just been putting the finish reamer in the finished chamber and checking for wobble and measuring any perceived movement with a .0001 indicator. I also used an inside mic to check the .200 line and compare it to what the reamer measures. I think I’ll be ordering some pin gauges too for a quick reference to see if it’s oversize by at least .0005.

 

[jackieschmidt]

Yes, but how do you know you're on the .200 line?


Either way... this is one of those "probably don't need to overthink it" things.

^^^^^^^^^

 

[FireMedic]

If you want to do it in the lathe….a quick way would be to use your DRO in conjunction with a indicator mounted in your tool post. Touch one side of the chamber and set zero, go to other side until it just touches and then add the ball tip thickness of the indicator. This will give you the diameter. It should be easy to find .200 on z axis also. You will need to adjust indicator up and down to find the center of the chamber first though.

 

[tonyben]

Okay, so what's the 200 line I keep seeing?

By the way, I'm using a JGS floating reamer holder (https://www.jgsprecision.com/pages/jgs-tools). I line up the barrel in the headstock using a spider at both ends and measure the first two to three inches of rifling for runout with my stylus or with one of those Gordy Gritters piloted grizzly rods.

This is a good setup, no? I also have a Bald Eagle reamer holder, but there are some cases where I like the JGS better, especially when I have a barrel that was reamed at the factory with a rougher and the body is too small for my finish reamer to go in far enough for the pilot to engage the rifling. In that case, It's nearly impossible to use my Bald Eagle reamer to stay straight with only a single point of contact on the shoulder and no pilot to keep it in alignment.

In that case, the JGS keeps the reamer body centered enough to get the finish reamer to cut straight and engage the rifling.

Tony.

 

[Tim Singleton]

The 200 line is .200" up from the base of the case

 

[tonyben]

The 200 line is .200" up from the base of the case

Thanks!

 

[Jud96]

edit

 

[Jud96]

I wanted to give a quick update after cutting a couple more test chambers and taking measurements a few different ways. I have found a technique that I like and is giving really repeatable results.

The quick and easy way to this is simple, measure the actual .200 line dimension of the reamer (ex. .4715) and pick the closest pin gauge to the .0005. It should go in, then take the next bigger size (ex. .472) and it shouldn’t go in or maybe just barely start.

The way I’ve found that I like and is a little more precise is as follows

- Take the pin gauge closest to the .200 line (.4715 in this example)

-Measure the overall length of the gauge (ex. 2.000”)

- Insert the pin into the finished chamber and measure from the back of the barrel to the gauge ( ex. 1.800”)

- If it’s a counterbored breach like a Remington you will have to subtract those two numbers (2.000 - 1.800 = 0.200). This shows the pin gauge is right at the .200 line.

- For a flat breach, you will have to take the same measurements (ex. 2.000”- 1.940”= 0.060) but then add the depth of the bolt face (ex. .060 + 0.140 = 0.200).

If you really want to get technical you can do the math with any clearances made between the barrel and bolt, but the above math will get you very very close.

In the above example it’s a perfect scenario where the pin gauge is the same diameter as the .200 line, thus the math works out that way. However, if the gauge was actually 0.300 deep in the above example, you could do some math to determine how oversize the chamber is.

Let’s pretend this is a .308 Winchester chamber. A .308 reamer typically has about .0116 per inch of taper. The taper per inch is shown on Manson’s drawings and can be determined with some math if it’s not called out on your reamer print. With the taper per inch given, doing some more math would show that every .010 of length would equal about .00012 of diameter. So if the .4715 pin gauge was sitting at the .300 line we could simply take .00012 x 10 = 0.0012. Add the .0012 to .4715 = .4727. Doing this math gets you very very close to determining the size of the .200 line and how oversize the chamber might be.

Sorry for the long winded post. I just thought I’d share what has been working for me. It’s less complicated once you do it a couple times. I know it’s not exacting or perfect, but it will get you within .0005 accuracy and I believe that’s about as good as you can repeatedly get.