Measuring Finished Chamber Dimensions

[DaveTooley]

This has been a very interesting thread with an experienced smiths and machinists contributing. I am at the beginning of the learning curve when it comes to running a lathe. My question as a beginner is: If you have measured the reamer and are confident it is sized within tolerance and it is a piloted reamer with a bushing that fits correctly, wouldn’t you just need to measure the diameter of the chamber at the .200 line with a gauge pin to detect if the reamer was inserted concentric with the bore in a stable setup? If the gauge pin hits the number at the called out dimension at the .200 line doesn’t that tell you the chamber was cut to size and not oversized as long as the pilot keeps the front of the reamer centered and snug?

We all started somewhere. For a long time I was dependent on the bushing keeping the reamer centered. That was before bore scopes and an even simpler tool, a jewelers loupe. There are tolerances in everything we use. Bushing and reamers included. So there is no such thing as a perfect fit. The simple key to a concentric chamber is having a concentric hole for the reamer to follow. That's radially and axially . Do that and you don't need a bushing. Then a simple pusher to push the reamer. It really is that simple.
By chambering this way the chamber is cut to the size of the reamer and there is really no need to check the chamber. Errors in reamers most frequently happen in the throat area. I have never heard of a problem with the body of a reamer. Maybe if you get to cute tightening up the dimensions. Very seldom are there problems with two manufactures we use, more frequently with another manufacturer. I stopped spec'n out the body of reamers a long time ago. Reloading dies have to work so I base everything off of SAAMI dimensions.

 

[LVLAaron]

Very seldom are there problems with two manufactures we use, more frequently with another manufacturer

 

[Skedaddle Shooter]

We all started somewhere. For a long time I was dependent on the bushing keeping the reamer centered. That was before bore scopes and an even simpler tool, a jewelers loupe. There are tolerances in everything we use. Bushing and reamers included. So there is no such thing as a perfect fit. The simple key to a concentric chamber is having a concentric hole for the reamer to follow. That's radially and axially . Do that and you don't need a bushing. Then a simple pusher to push the reamer. It really is that simple.
By chambering this way the chamber is cut to the size of the reamer and there is really no need to check the chamber. Errors in reamers most frequently happen in the throat area. I have never heard of a problem with the body of a reamer. Maybe if you get to cute tightening up the dimensions. Very seldom are there problems with two manufactures we use, more frequently with another manufacturer. I stopped spec'n out the body of reamers a long time ago. Reloading dies have to work so I base everything off of SAAMI dimensions.

Thanks Dave,
I have been starting the chamber with a piloted drill, then come back with a boring bar to true any inconsistencies in the pre drill. Then the finish reamer and so far that has worked well for me. I appreciate your input.
Joe

 

[Skedaddle Shooter]

How do you accurately determine where the .200 line is? And how do you measure it?


To answer some of your questions with other data... Some chambers are oval. Some are octagon shaped. If they're oversized they're probably an odd shape. How would you measure those?

Just some food for thought.

Good points, not having used any of these methods I can’t speak from experience. It seems to determine the position of the .200 line you could turn a pin to the dimension of the chamber at the .200 line, hold it in your tail stock. Run it in until you touch the back of your go gauge and zero the tail stock. Account for the difference between the length of the go gauge and your chamber (go + .002”?) and advance the pin forward and see where it contacts the chamber walls. If it exceeds the .200 line you may have a problem.
My original thought was how will the OP determine if the chamber is cut oversized. I assumed if it were over sized it would be a result of his reamer being off the centerline of the bore, leaving an oversized but symmetrical chamber. I have no idea how to measure an octagonal chamber, that would be a tough oneI was just thinking on how to confirm you had bored the chamber to the correct size, but you are correct, an odd shape boring could still contact the pin at the .200 line in a couple of spots giving you the allusion of a properly reamed chamber.

 

[Tod Hendricks]

How do you accurately determine where the .200 line is? And how do you measure it?


To answer some of your questions with other data... Some chambers are oval. Some are octagon shaped. If they're oversized they're probably an odd shape. How would you measure those?

Just some food for thought.

Wouldn’t sweeping the chamber with a dti post chambering confirm an odd shape?

 

[LVLAaron]

Good points, not having used any of these methods I can’t speak from experience. It seems to determine the position of the .200 line you could turn a pin to the dimension of the chamber at the .200 line, hold it in your tail stock. Run it in until you touch the back of your go gauge and zero the tail stock. Account for the difference between the length of the go gauge and your chamber (go + .002”?) and advance the pin forward and see where it contacts the chamber walls. If it exceeds the .200 line you may have a problem.
My original thought was how will the OP determine if the chamber is cut oversized. I assumed if it were over sized it would be a result of his reamer being off the centerline of the bore, leaving an oversized but symmetrical chamber. I have no idea how to measure an octagonal chamber, that would be a tough oneI was just thinking on how to confirm you had bored the chamber to the correct size, but you are correct, an odd shape boring could still contact the pin at the .200 line in a could of spots giving you the allusion of a properly reamed chamber.

I think a ball bearing would be better. But you'd also need someone who's better at math than me to tell you how to measure it

 

[tobnpr]

Good points, not having used any of these methods I can’t speak from experience. It seems to determine the position of the .200 line you could turn a pin to the dimension of the chamber at the .200 line, hold it in your tail stock. Run it in until you touch the back of your go gauge and zero the tail stock. Account for the difference between the length of the go gauge and your chamber (go + .002”?) and advance the pin forward and see where it contacts the chamber walls. If it exceeds the .200 line you may have a problem.

Or
Just cast the chamber and measure it...jeez...

 

[Rich S]

I’d use an indicator mounted on the cross slide. Pick up a know OD on the tennon, zero the indicator and DRO then pick up the ID I want until the indicator is zeroed then calc off the DRO. You can also measure other IDs in the chamber such as neck diameter that way. Pretty quick and easy if the barrel is still in the lathe.
You are limited by the accuracy of the indicator and DRO.

 

[liljoe]

For the people saying to just make a chamber cast. The OP is going to be doing this for business so doing a cast for every one is not really feasible.
Joe

 

[clowdis]

I think that if I had the need to check a number of barrel chambers for dimension or consistency I would use steel balls. Drop one into the chamber and measure the distance from the back of the barrel to the ball. You could get the actual dimension by this method or for a speed check you can check a number of chamber rather quickly by knowing what this dimension should be.

 

[Tim Singleton]

There really isn’t much to be concerned about as far as an over sized chamber that would require you to verify each one. UNLESS your setup allows you to pickup a little bit of chatter
That is BAD and undoubtedly will give you an oversized chamber

 

[Toolbreaker]

Jud96, something like the attached pic makes for quick on-the-machine gaging of tapered holes. A drop indicator base relieved in the center section for an off the shelf full-diameter ball indicator tip. A gage ball and depth mic will accomplish the same thing, just a little less convenient. Something like this can be zeroed from a surface plate or any known flat.

The important part is that the included angle of the taper has to be known. In the case of a chamber, that means knowing the as-measured taper of the reamer used.

I tried to type out the mathematical steps involved to trig the depth at the tangency point of the ball radius, but failed miserably. I couldn't even track what I had just typed . If you have access to any CAD software, it's a very simple calculation after the data is input.

Dave Tooley is correct, though. Gage balls are the most definitive way to check this type of feature. One appropriate size gage ball can check for diameter at a known angle, 2 different sized balls for diameter and taper angle both, and 3 different balls for the above, plus linearity of the taper. That part is really getting down deep into the weeds.

All of this might be overkill for you, though. Just depends on what you're after.

 

[Jud96]

Thanks everyone for the input. This has been a good thread and I’m happy with the information shared.

I have been trying a couple things to check finished dimensions. I have been using the inside mic more and getting repetitive results measuring the base. I also took the reamer with no bushing and put it in the finished chamber and felt for any wiggle, then put a .0001 Interapid indicator on one of the cutting flutes and tried to wiggle the reamer and basically had no movement. Then I rotated the spindle opposite of the cutting direction and checked each flute and all were within .0002 of “0” on the indicator.

I think I’ll pick up some pin gauges in .0005 steps and try those for a quick check. I may also get some 3 point bore mics to play with and compare to my other methods. However, I’m pretty happy with the inside mic and using the reamer itself to check the finished chamber for it being oversized.

 

[Sniperhandle]

3 point bore mics will be a good investment for more than just chambers.

 

[LVLAaron]

3 point bore mics will be a good investment for more than just chambers.

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.

 

[Sniperhandle]

I could care less about the 200 line, I just know that the 3 point mics are a good investment regardless, If I dial things in perfectly straight before I ream, I know my chamber is going to be tight.

Interesting points brought up by everyone though.

 

[LVLAaron]

I could care less about the 200 line, I just know that the 3 point mics are a good investment regardless, If I dial things in perfectly straight before I ream, I know my chamber is going to be tight.

Interesting points brought up by everyone though.

Funny story...

I started doing barrels for a guy. His reamer. He was having all sorts of extraction issues (308) tried all sorts of brass, small base dies, new brass, old brass, everything. He comes over, shows me all of his science experiments. I send him home and ask him to get a reamer print. Turns out the base was pretty tight; .4702 if memory serves.

Long story short... his old guy was making sloppy chambers for him for 10 years. I made him a precise chamber and it caused issues. Ran one of my FTR reamers in by hand (had a small neck) and cleaned him up in 5 minutes.

I ask for a print now if someone is bringing me a reamer.

Another story that's kind of the opposite... One of my guys shoots probably 10k rounds a year. He too was having all sorts of extraction issues. Couldn't figure it out; we know it's not the chamber as he goes through barrels like beer on a friday. He ended up sending his sizing die back to RCBS and got a response like "Wow, this thing is very badly worn, we're sending you a new one"

 

[FarmallSM]

Never occurred to me to want to know that. I trust the JGS or Manson reamer print. To Mr. Tooleys point, earlier post, make sure chamber is running concentric to bore. I dial bore in to a bare minimum of .0002 if not “0” runout. I check for runout several times during chambering. I chamber with a JGS floating reamer holder and exact size bushing on end of reamer. I use a bore scope and check for chatter several times, making sure there is none. Bore scope also allows me to check polishing of the chamber. One of the last measurements is to use an indicator and check for runout of the finished chamber. I fire every one of my rifle builds. Checking for function and small groups. Once I started using Bartlein or Krieger barrels, a lot of my issues went away.

 

[Jud96]

Never occurred to me to want to know that. I trust the JGS or Manson reamer print. To Mr. Tooleys point, earlier post, make sure chamber is running concentric to bore. I dial bore in to a bare minimum of .0002 if not “0” runout. I check for runout several times during chambering. I chamber with a JGS floating reamer holder and exact size bushing on end of reamer. I use a bore scope and check for chatter several times, making sure there is none. Bore scope also allows me to check polishing of the chamber. One of the last measurements is to use an indicator and check for runout of the finished chamber. I fire every one of my rifle builds. Checking for function and small groups. Once I started using Bartlein or Krieger barrels, a lot of my issues went away.

I’ve gotten reamers that were out of tolerance, so I check everything. Also, if the chamber is big it won’t show up on an indicator if the chamber is round but oversize. In theory everything should be perfect with a good setup, but it’s the same reason you check everything else. Firing the rifle and measuring the fired brass is also a valid method, I just wanted to check before the barrel leaves the lathe.

 

[FarmallSM]

I’ve gotten reamers that were out of tolerance, so I check everything. Also, if the chamber is big it won’t show up on an indicator if the chamber is round but oversize. In theory everything should be perfect with a good setup, but it’s the same reason you check everything else. Firing the rifle and measuring the fired brass is also a valid method, I just wanted to check before the barrel leaves the lathe.

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