Indicating Rifle Barrels

[Hillbilly]

I have been having good results indicating rifle barrels in my 1963 South Bend Heavy 10 lathe using Buck Ajust-Tru chucks. I use 312B-15 Interapid .0005 indicator with 2.75 stem. I also use the Mitutoyo 513-504 .0001 round body as per Jackie and Butch. I had a machinist tell me the other day that the indicators dont really work well on interrupted surfaces like a rifle bore. He thinks using pins or rods is the more accurate way. What are your thoughts on this?

 

[Wayne Shaw]

In order to use a pin, it has to have clearance. No clearance when reading directly to the surface.

 

[SGS]

I have been having good results indicating rifle barrels in my 1963 South Bend Heavy 10 lathe using Buck Ajust-Tru chucks. I use 312B-15 Interapid .0005 indicator with 2.75 stem. I also use the Mitutoyo 513-504 .0001 round body as per Jackie and Butch. I had a machinist tell me the other day that the indicators dont really work well on interrupted surfaces like a rifle bore. He thinks using pins or rods is the more accurate way. What are your thoughts on this?

Indicating directly on the bore is the most accurate method. There can be issues that degrade the quality of the indicator reading. The best arrangement is to use a large enough and slick enough ball tip so that it does not jump around when climbing the lands. Good quality indicators like the ones you are using also help. There is a certain amount of refined technique to getting the best reading. If you are indicating the groove diameter, rock the chuck back and forth slightly at the low point to get the most consistent reading.

Certain types of machine work, primarily rework, allow a machinist to develop the skill of accurately positioning a work piece even if the reference surface is interrupted, out of round, or damaged.

 

[Hillbilly]

Thats my take on this as well. I have used PTG range rods and zero on it and then run indicator inside the barrel to find .001-.002 of runout. I feel like using the indicator inside the bore is the best and most accurate way.

 

[Jim Kobe]

Does it get a little inconsistent when indicating a 6-groove vs a 4 or 8 groove in a four jaw?

 

[Lee Martin]

I initially indicate with a Deltronic pin in each end. That gets me real close. Then I'll zero on every groove. More often than not, the Deltronic indication is within a ten-thousandth or two.

-Lee
www.singleactions.com

 

[JerrySharrett]

Does it get a little inconsistent when indicating a 6-groove vs a 4 or 8 groove in a four jaw?

When dialing in odd numbered flute barrels use a close fitting reamer pilot bushing and indicate the bore of that bushing. Over the years we've had 3-groove, 5-groove and 2-groove barrels to chamber and using the bushing bore works great.

Think about it. the truest part of a rifle bore is the lands not the roves. The lands surface is where the barrel was drilled, reamed and lapped. The grooves are cut afterwords, sometimes one at a time, AND the first thing the bullet engages is the lands.


.

 

[Zebra13]

When dialing in odd numbered flute barrels use a close fitting reamer pilot bushing and indicate the bore of that bushing. Over the years we've had 3-groove, 5-groove and 2-groove barrels to chamber and using the bushing bore works great.

Think about it. the truest part of a rifle bore is the lands not the roves. The lands surface is where the barrel was drilled, reamed and lapped. The grooves are cut afterwords, sometimes one at a time, AND the first thing the bullet engages is the lands.


.

Ditto the above. When doing 3 groove Liljas, I've always used reamer bushings. Never found the need for gage pins. What I don't know, and have never checked, is concentricity between the ID and OD of the bushings. I've always done it with a bit of bushing sticking out the bore and run the stylus on that. I THINK I read somewhere that the OD and ID are held within a tenth.

I feel the same way Jerry does about the lands being the most accurate surface to measure from. I don't think you'd ever be able to tell the difference on paper, though.

Justin

 

[Pete Wass]

I agree with this

When dialing in odd numbered flute barrels use a close fitting reamer pilot bushing and indicate the bore of that bushing. Over the years we've had 3-groove, 5-groove and 2-groove barrels to chamber and using the bushing bore works great.

Think about it. the truest part of a rifle bore is the lands not the roves. The lands surface is where the barrel was drilled, reamed and lapped. The grooves are cut afterwords, sometimes one at a time, AND the first thing the bullet engages is the lands.


.

makes all kind of sense to me.

Pete

 

[Hillbilly]

Jerry i have used your reamer bushing trick many times on 5 Groove barrels to help me set them up. It works so well that if i put a indicator back inside the bore its within .0002 most times!

 

[HiWall]

When dialing in odd numbered flute barrels use a close fitting reamer pilot bushing and indicate the bore of that bushing. Over the years we've had 3-groove, 5-groove and 2-groove barrels to chamber and using the bushing bore works great.

Think about it. the truest part of a rifle bore is the lands not the roves. The lands surface is where the barrel was drilled, reamed and lapped. The grooves are cut afterwords, sometimes one at a time, AND the first thing the bullet engages is the lands.


.

I agree that the tops of the lands would be the most uniform in diameter as compared to the cut-grooves. However, with concerns towards bullet alignment, I'm not so sure that the lands would be more instrumental to the bullets alignment in the bore than the grooves. This would certainly be the case for obturating lead bullets. Think of an example where the grooves were not cut to a uniform depth and were cut so that they formed a circle who's center was off-set from the center formed by the tops of the lands. Upon firing the lead bullet would conform to the grooves whereas the lands would just cut deeper into one side of the bullet. The center-line of the bullet would still be off-center to the center formed by the tops of the lands. I agree this is an extreme example and used to show my point.

If the bullet were a jacketed bullet, being harder and somewhat non-obturating, it might perhaps try to stay aligned more with the tops of the lands in the example above with possibly gas-cutting resulting where the grooves were cut deeper.

In practice, we can certainly indicate the tops of the lands as well as the grooves and thereby check their orientation to one another and decide then which is better to use to align the barrel in the lathe.