Barrel thread concentricity and alignment


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CoRoMo
April 18, 2012, 07:11 PM
This comes from the context of my previous poll thread -> http://www.thehighroad.org/showthread.php?t=652138

How would a layman like myself check a threaded barrel to make sure the manufacturer/gunsmith did a proper job?

I've never actually double-checked my barrels' threads. I've always trusted that the thread work was done correctly, and apparently it has been, because I've had no issues.

But I HAVE met a number of "gunsmiths" that forced me to be suspect of their handiwork, knowledge and skill. This has caused me to do most all of my own work for the last several years and I'm actually quite competent IMHO.

So... the next time I buy a barrel or have one threaded, how should I go about double-checking it?

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Ludasmith
April 18, 2012, 08:55 PM
I'd think you could measure from the inside of the barrel to the edge of the threads with a caliber? If it's the same distance in 3 spots it should be centered. I have no idea what an allowable tolerance would be though.

CoRoMo
April 19, 2012, 11:37 AM
That makes sense, but I suppose the one that I'd be most concerned with is whether or not the threads and the barrel face are perfectly square with the bore line.

For reference, here's a good image of suppressor mounts that I found: http://www.canadiantactical.ca/Images/SuppressorMountTypes.JPG

Captains1911
April 19, 2012, 02:13 PM
Attach can, look down the bore from the receiver end, if it looks ok it probably is.

CoRoMo
April 19, 2012, 03:00 PM
Probably.

I hate 'probably'.

:D

wally
April 19, 2012, 10:07 PM
Gemtech recommends drill rod of the appropriate diameter to put in the bore and suppressor to see if it touches anything when rotated. 19/64" for 7.62 can.

CoRoMo
April 23, 2012, 11:46 AM
Mainly, I was wondering what would occur if I took a barrel to a skilled gunsmith or machinist and asked them to check it's bore concentricity and the barrel face alignment. Would they do more than looking at it or inserting a dowel?

I'd hope so, but I've asked a "gunsmith" to check some specs on a gun before, he glanced at it from an arm's length and stated that everything was in check.

highorder
April 23, 2012, 01:33 PM
Mainly, I was wondering what would occur if I took a barrel to a skilled gunsmith or machinist and asked them to check it's bore concentricity and the barrel face alignment.

Machinist here.

I would mount your barrel in a 4 jaw chuck and indicate the bore center. I would then move my dial indicator and inspect the muzzle face and thread OD for concentricity, circularity, and cylindricity.

But first, I attach your suppressor and spin it to see how it looks. ;)

You need an accurate pair of centers (lathe, bench QC gauge, etc), an indicator, and something to measure like a caliper or micrometer.

wally
April 23, 2012, 02:17 PM
You need an accurate pair of centers (lathe, bench QC gauge, etc), an indicator, and something to measure like a caliper or micrometer.

What he said.

I just cut and threaded my Uzi barrel. To verify the thread cutting I did the free end I was going to cut off by simply cutting the threads using a three jaw chuck, screwing on the 9mm piston for my Osprey 45 it was easy to see the exit hole of the barrel was not centered, probably 0.008-9" off.

Using a four jaw chuck and indexing off the groves (figured there'd be less wear there) I got the peak-to peak run out about 0.001 in the grooves (the lands were about 0.004-5 high if I recall correctly).

Now looking at the bore through the piston there is no visible deviation form center.

The Uzi was easy since its barrel comes off for cleaning, a normal rifle will probably need a lot of disassembly before you can put the barrel in a lathe for threading.

CoRoMo
April 23, 2012, 05:21 PM
Now that's what I'm talking about!

I had guessed that a lathe would come in handy, and I have access to one. I just didn't know if taking apart this rifle and chucking the barrel would be worth the effort if there was an easier way.

Ludasmith
April 23, 2012, 10:23 PM
Glad some knowledgeable folk chimed in. I woulda checked it my way and probably screwed it up haha

alemonkey
May 6, 2012, 11:58 AM
I'm just learning the mysteries of lathe operation, and I just threaded my first .22 barrel. This is a little different than checking concentricity on a barrel that's already threaded, but what I did was turn a piece of brass bar stock down to be a tight slip fit in the bore. Then I chucked the barrel up in a 4-jaw chuck and indicated off the brass rod. I cut the barrel down and cut a crown, turned it down to the correct diameter, and threaded it, and I rechecked concentricity between each step. I ended up with less than .001" of runout. Not sure if that's the standard way of doing it, but it seemed to work for me.

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