Reducing barrel shank thread diameter

Blackthorn762 · Jul 3, 2008

Hi all,

While not at all new to guns and doing minor repairs, I am new to doing machine work in general and with guns in particular. I am currently planning out a benchrest project which involves mating a Turkish Mauser action with a Norwegian manufactured Mauser barrel chambered for .308. This is easy enough to do, though I need advise on the best way to reduce the thread diameter on the barrel - it will fit any standard Large Ring action (1.10 shank thread diamameter, 12TPI) - to fit the Turkish Mauser action, which is considered a Large Ring action, though threaded to Small Ring specs (.980 shank thread diameter, 12TPI).

So...what is the best way to reduce the shank thread diameter from 1.10 to .980 while retaining pitch and thread shape (60 degree V)?

Any suggests are greatly appreciated!

garyphansen · Jul 3, 2008

You will need a thread cutting lathe. Gary P. Hansen

Blackthorn762 · Jul 3, 2008

Have one...it's an Atlas 10F...just not all that familar with it's operation just yet. The shank threads are already cut, I need to know the best way to reduce the current thread diameter without reducing the diameter of the shank itself.

deltaenterprizes · Jul 3, 2008

It will be harder and take longer than if you started with a blank shank on the barrel. After reducing the diameter .120 there will still be some of the old threads left aprox .060deep and to do it right you will have to capture the old threads so that the tool will cut in that old groove,pain in the arse.
You also need a depth mic to measure for the length of the shank.You will need a finish reamer and a set of go no/go guages to set the headspace.
This needs to be done on a lathe with thread cutting capabilities by an experienced operator.

This is not the best beginner's project.

I would get a large ring action and go from there

Clemson · Jul 3, 2008

Mount the barrel between centers, and cut the tenon down to 0.980 inches. That will clean up the existing threads entirely. For best accuracy, the tenon length needs to be the same as the distance from the primary torque shoulder (inside shoulder) of the receiver to the (secondary torque shoulder (outside shoulder). Some folks make the tenon longer so that the barrel does not bear on that secondary shoulder at all. The Turk will have significant bearing surface on both, so I like to make the tenon the same length so that both shoulders bear. Cut a relief groove on the tenon, and then cut the new threads at 12 tpi. The thread form, by the way, is Whitworth, which is 55 degrees rather than 60 degrees. A 60 degree thread can be made to work, but 55 is better.

We could run an entire thread on how to hold the barrel in the lathe, but an alternate to threading between centers is to chuck the muzzle end and use a center on the breech end. The length of the barrel pretty much eliminates any problems from fixing one end in an imperfect chuck.

Clemson

Blackthorn762 · Jul 3, 2008

I have the lathe with the thread cutting capabilities, all of the measuring mics I will ever need, and access to the reamer, go/no-go guages...all I lack is the "experienced operator" part.

I guess what I really need to know is how to go about doing the actual thread diameter reduction.

Once I know how that works, I can practice on old tube stock by cutting original diameter threads and then reducing them until I get it right.

cheechako · Jul 3, 2008

black

All of the responses have given you everything you need to know. If you still do not understand what needs to be done then you probably are not ready to undertake a barrel project. Fitting a Mauser, in particular, takes an understanding of the relationship of the two shoulders. The possibility that you may have to make adjustments to the chamber requires a different set of skills altogether (plus the necessary reamers and gauges).

Before taking on a job like this I would suggest that you get either an old barrel stub or simply a piece of round stock (even brass will work) and do the job a couple of times for practice. Once you appreciate what has to be done to get the correct result you'll be ready to try it for real.

But, (and I'm not being a smart a$$) I hope that the .308 barrel is expendable.

Ray

Blackthorn762 · Jul 3, 2008

cheechako said:
black

All of the responses have given you everything you need to know. If you still do not understand what needs to be done then you probably are not ready to undertake a barrel project. Fitting a Mauser, in particular, takes an understanding of the relationship of the two shoulders. The possibility that you may have to make adjustments to the chamber requires a different set of skills altogether (plus the necessary reamers and gauges).

Before taking on a job like this I would suggest that you get either an old barrel stub or simply a piece of round stock (even brass will work) and do the job a couple of times for practice. Once you appreciate what has to be done to get the correct result you'll be ready to try it for real.

But, (and I'm not being a smart a$$) I hope that the .308 barrel is expendable.

Ray

I got interrupted while writing my last post and never saw the other ones explaining the procedure I was asking about IN the last post. When I finished writing and posting, the order of the posts made it appear that I didn't understand...lol Yet more proof that timing is everything...

Having read the posts in the current order it all makes much better sense now...based on what I read, you are right however, at my level of experience it is more prudent for me to only do this project with some direct guidence from someone with a few years in this area. I guess the Turk .308 will have to wait for now. I do have a Yugo action, maybe I'll give that conversion a shot. All that will require is headspacing, which I DO have experience with.

Thank you all for your help and advise!

GGaskill · Jul 5, 2008

The only tricky process here is picking up the remnant of the original thread after reducing the shank to .980". And if the original threads were made with the proper thread form, there will be so little of them left that picking them up will be more of an exercise in precision than something that is really necessary.

As an experiment, I suggest threading a piece of scrap to 1.1-12 using a properly shaped threading bit (including the flat on the end), then turning the thread down to .980" diameter and seeing how much thread is left. It will give you practice threading as well as illuminating this subject.

gnorbury · Jul 6, 2008

I suppose the first question is whether you know how to turn threads up to a shoulder? If not, I really recommend you practice on some bar stock first, otherwise you'll probably wreck the barrel.

Thread depth for 12tpi Whitworth is 0.0533" according to the Machinery Handbook, so you should be able to simply turn down the shank to 0.98, and re-thread from scratch.

cheechako · Jul 6, 2008

Excuse me guys, I'm a nit-picker, and can't help myself any longer. The term "shank" was used years ago and has fallen out of favor by today's gunsmiths who prefer the term "tenon". As I said, I'm a nit-picker.

Ray

aametalmaster · Jul 6, 2008

I did mine for my old GEW 88 which was also .980 and i started with a 1.10 barrel i got for free. But i have done enough threading it was fairly easy to pick up the start. I would also practice on some barstock that was threaded 1.10 and turned to .980 to get the hang of it. And i thought mine was a 55 degree thread also...Bob

Jay Idaho · Jul 6, 2008

Practice first,

duplicate the 1.1 diameter thread as close as possible and THEN reduce that as necessary to get your desired diameter and cut the new threads. By then, you will be a lot more confident in the actual task.

huntinguy · Jul 7, 2008

"Jay, Idaho
duplicate the 1.1 diameter thread as close as possible and THEN reduce that as necessary to get your desired diameter and cut the new threads. By then, you will be a lot more confident in the actual task. "

Jay has it right. I would suggest: once the duplicate threaded section is done. Take it out of the lathe, put it back in and chase the threads down. It will give you an idea of what you are up against.

It is not a complicated task, machinists chase threads all the time.

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