Gunsmithing Barrel to Action Threading Question

[scotharr]

I just read the Vaughn book about his accuracy experiments where he identified barrel/action thread movement as a source of inaccuracy and the proposed a different type of threading to improve the lockup and make more threads actually engage and provide support.

Are there any practical implications to this? Does anybody do anything different to provide a more consistent mating between barrel and action?

For example, the surgeon action claims a longer tenon length as a benefit. But if only the first couple threads bear all the torque, why would it matter?

Thanks,
Scooter

 

[our gang]

Re: Barrel to Action Threading Question

Interesting stuff but it leaves me wondering why Savages with no threads at all, shoot so well...... Also Sauer 202s are usually sub MOA rifles with interchangable barrels and a hunting design.

 

[Hink]

Re: Barrel to Action Threading Question

I believe the issue being addressed is the relative movement experienced between the barrel and action during and following a shot. Any movement in this area with a threaded interface will affect the point of impact of the following shot.

As far as interchangeable barrel systems...there would (I think) be some method of retaining the barrel to the action in such a way that either there is no relative movement of the barrel and action or the two are allowed to move but return to battery exactly as they were oriented prior to the intial shot.

 

[sobrbiker883]

Re: Barrel to Action Threading Question

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Mr. Humble</div><div class="ubbcode-body">Interesting stuff but it leaves me wondering why <span style="color: #3366FF">Savages with no threads at all</span>, shoot so well...... </div></div>

Funny, the last four or five I had all had threaded barrels that use a locknut to both fix the barrel and suck up the slack in the threads setting headspace..........

 

[rockybrashear]

Re: Barrel to Action Threading Question

with a properly threaded and seated barrel the torque is spread throughout the entire threading.in reality this isnt the case in most production rifles. savage tackles this problem with the lock nut buy stretching the barrel between two points. the receiver and the nut.this works to bold the barrel securely. again in reality though most rifle manufacturers very closely control tolerances it is true that the barrel reciever threading is a weak point due to insimilarities between them.

 

[scotharr]

Re: Barrel to Action Threading Question

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: savage7mil</div><div class="ubbcode-body">with a properly threaded and seated barrel the torque is spread throughout the entire threading.in reality this isnt the case in most production rifles. </div></div>

that was Vaughn's point, only the first few threads engage the action and take all the torque. This, in turn, permits some movement under recoil causing measurable inaccuracy. He then went on to propose a new thread-type that would engage all the threads, thereby tightening up this joint.

That said, I have not seen anyone (besides savage?) address this issue and that is my question. May have to go to the pointy-headed benchrest column with this question

 

[Rafael]

Re: Barrel to Action Threading Question

I am very happy with my Savages, but I seriously doubt there is any advantage to the nut system other than these two items:
1-The ability to manufacture barrels and actions with little regard for shoulder location...any barrel will mate with any action on assembly.
2-The ability for the end user to order aftermarket barrels without having to send the action to the smith.

If anything the nut makes for a secondary threaded area allowing more flex, and it also sharply reduces the taper of the barrel to a smaller diameter(just forward of the threaded area) than a comparable taper for a shouldered barrel. The threads mating to the action are going to have the very same flex in a nutted or a shouldered action, but the Savage adds more flex in the additional thread mating between the nut and the barrel.

When the two systems are compared side-by-side, I can't imagine the Savage system being near as strong or stiff as the shoulder system.

Of course, I am no engineer.
Would love to hear an analysis from one of them.

 

[BBeyer]

Re: Barrel to Action Threading Question

Most of the BR shooters that I shoot with seem to think that a properly threaded barrel is all that is needed to retain the accuracy of a rifle. I have a number of actions and about twenty barrels for them, any one of the barrels will screw into any of the actions [as long as it is the same size action] and shoot well enough to win a few of the score matches that we have in New England. I have never used any kind of torque wrench on any of them. Just snug them up well with an action wrench and good to go. Hi RAF, I"ll give you a call early this week.

 

[Rafael]

Re: Barrel to Action Threading Question

Looking forward to it, Bob!
It's good to hear from you.

 

[HPitts]

Re: Barrel to Action Threading Question

The threads on a Remington 788 barrel is about 1.250 long and most of the time they are tack drivers .

Howard

 

[bohem]

Re: Barrel to Action Threading Question

Engineering Perspective:

1) Improperly mated threads will load the first few threads. In fact, a FEM analysis I did during grad school showed that with a decent quality machined thread like you find on AN grade fasteners, only 3 threads carry all the load.

This is easily shown when you strip out a nut, the first threads to go are the leading ones and it propagates down the nut as it tightens up

2) Making the threads all mate and load up "equally" isn't quite going to work just because the threads closest to the should will carry more tensile or compression loading at first. When the rifle undergoes a stress cycle the deflection will load the other threads on order down the tenon ring. If there's a crappy fit on the tenon threads the primarily loaded ones need to deflect more before the others come into play.

Having a longer tenon like Surgeon uses adds loading radius to the threads. It doesn't really make a difference for something like a tensile load on the barrel/receiver joint. It makes a difference for "barrel flip" or a bending load. Think of it akin to putting a 30" barrel on an action for open sights. You know a 20" tube is stiffer, but that extra 10" helps by adding additional sight radius and makes errors in POA more pronounced, thus making it easier to hit.

This is the similar idea to adding bending stiffness to the joint.

I haven't read the book you're talking about, I'm just telling you how I've seen and calculated that bolted joints will act under a bending moment.

 

[300sniper]

Re: Barrel to Action Threading Question

i am not an engineer nor have i ever taken any classes on the subject. i am wondering if a larger outer diameter of the shoulder would have more affect on the stiffness of the joint than a longer threaded tenon.

 

[bohem]

Re: Barrel to Action Threading Question

The farther out that you can move the load carrying surfaces during the bending load, the stiffer the action will be.

So yes, 300, the farther out you get the edge of teh shoulder the better off you will be. Adding length to the tenon threads and making the OD of the barrel shoulder will add stiffness to the joint.

 

[rijndael]

Re: Barrel to Action Threading Question

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: bohem</div><div class="ubbcode-body">only 3 threads carry all the load.</div></div>

What about folks that use Sprialock threads?

I know at least 3 shops that use them for barrel tenons.

 

[bohem]

Re: Barrel to Action Threading Question

I haven't done an analytical study on spiralock thread inserts so I can't give you a detail on those, sorry.

 

[Clark]

Re: Barrel to Action Threading Question

Vaughn sees the tension of the barrel tennon being relieved during firing, allowing movement as a problem.
He sees the thermal movement of the barrel tenon threads at the front of the receiver as a smaller problem.

Vaughn's proposed redesign [not existing hardware modification] solution is to cut the receiver to form an inner ring, like in a Mauser. He proposes a separate threaded retainer piece rather than use the end of the barrel tenon like a Mauser. That might save some manufacturing cost over the Mauser.

a) The Rem 700 has the tenon in tension at the front.
b) The Sav 110 has the tenon in tension at the front.
c) Vaughn proposes a rifle with a retainer and a barrel flange feature both in compression at the rear against the inner ring.
d) The Mauser has the tenon in front in tension, but less than the compression of the tenon at the rear against the inner C ring, when the tenon is cut 0.0015" long the way the Germans did it. Some recent Mausers have a back to back inner double D ring to reduce costs.
e) The 91/30 has the tenon in front in tension, and compression of the tenon in the rear from 1891 to 1942, but from 1942 to 1965 the inner C ring was eliminated for cost savings.

Vaughn seems to get good enough groups when he tightens up the barrel to receiver connection to 21,900 pounds of final pre load axial thrust.

This shot to shot trouble may be why Rem puts that glue on the threads and then tightens them up.

 

[Hateca]

Re: Barrel to Action Threading Question

With today’s machining and custom builds would it really make any difference?

It might if you are comparing factory to factory but I not sure you would see any mechanical difference if you are going to compare a new factory type thread design to a custom build using the same old threads of today. I'm darn sure the average shooter would never see the difference.

Haven't read the article but seems just another article suggesting a fix for something that really isn't a problem. If it were I think it would already have been done and become the norm if it really fixed an issue.

 

[scotharr]

Re: Barrel to Action Threading Question

read the book. He is saying that standard thread only load the first couple and that is the problem as mentioned above. Custom actions don't change anything if they use the same threads. thanks for the posts.

Here is a long post on the subject from the gurus of accuracy: http://www.benchrest.com/forums/showthread.php?t=59883 After reading, it became clear that this is a real issue and tightening the threads as much as possible will improve consistency/accuracy. Problem is standard threads can't take over 200lbs of torque. In my mind, that means the answer lies in creating a more efficient thread where more than just the first couple threads get loaded. I think that is the same conclusion Vaughn came to, but sometimes he lost me. They discuss spiralock in this thread and say it did not work well. From what I can gather, the current "best practice" is to torque to at least 100ft-lbs.

 

[Hateca]

Re: Barrel to Action Threading Question

I understand his point and agree with the notion that only the first few threads take the majority of the load when torqued but is changing the threads going to show a big difference in anything? I would say not unless someone is prepared to show significant advantages to changing what is currently being done.

Problem with that is there are too many other variables involved to even begin to discuss regarding whether two seemingly identical rifles will shoot better then the other, one with new thread design vs. old thread design.

 

[Alan Warner]

Re: Barrel to Action Threading Question

Vaughn never defined the tolerances of the thread fits he accused of causing the problem, number one, or their respective torque spec. If I remember correctly, he jumped to a conclution with a "solution" to a problem of undefined parameters.

Want a more riggid joint? Make a shouldered taper like a Cat or BT spindle tool holder. My bet is it will not show improved performance over a well fitted conventional thread. Of cource, your receiver ring will need to be twice the diameter of what they are now to accomdate the forces of a taper.

you are pissin in the wind here a bit.

alan

 

[scotharr]

Re: Barrel to Action Threading Question

Well, I think I got my answer from the BR thread:
1. torque it to at least 100 ft-lbs.
2. The threads should be able to withstand several hundred foot-lbs with no damage.

This was important to me because I'm building a switch barrel rifle where it was suggested to only torque it to 50 ft-lbs to reduce wear and tear. Given the threads should be able to handle 700-800ft-lbs of torque, I'll just torque it to 100 and call it a day.

 

[bohem]

Re: Barrel to Action Threading Question

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Scooter-PIE</div><div class="ubbcode-body">Well, I think I got my answer from the BR thread:
1. torque it to at least 100 ft-lbs.
2. The threads should be able to withstand several hundred foot-lbs with no damage.

This was important to me because I'm building a switch barrel rifle where it was suggested to only torque it to 50 ft-lbs to reduce wear and tear. Given the threads should be able to handle 700-800ft-lbs of torque, I'll just torque it to 100 and call it a day. </div></div>

If you want to find out your thread's load carrying capacity there's an easy formula to follow in "Handbook of Machinery"

A Mauser k98 has a pretty hefty 1" thread in it, from previous experience working on fastening tools that thread should take upwards of 1000 ft-lbs for several thousand cycles before having issues.

700-800 ft-lb is easy for a Remmy 700 sized thread.

Torque = k * F * dia

Rearrange that to F = T/(k*dia) and figure out what you're getting for a preload in the joint. THAT is the key to a joint's strength and stiffness. For threads like this that have no lubrication on them your k value should be about 0.2

On a switch barrel, if you're greasing the threads, drop it to about 0.1

Make sure that the load you're putting on the joint (F) is at least 10% of the bolt face load.

Sample calculation would be:

473 shell base at 60,000 psi --> 10,542 lbs

Min preload --> 1054.2 lbs

Thread is a 1" maj. diameter

Therefore, min torque is 210.9 ft-lbs

If your threads can take 800 ft-lbs without issue, and you're loading it at a 210 ft-lbs level you have unidirectional, cyclic loading with an R=0.0 value (goes from no stress to full positive stress)

You're about FS = 4, figure it's going to get switched 5000 times (yeah, I know that's WAY high) and see if you're within the stress level for the steel. I think you'll be fine personally.

If you greased the threads, it's now double that.

Lesson? Don't grease the threads.

 

[Clark]

Re: Barrel to Action Threading Question

bohem,
Great post.
The clamping force would be a function of the torque, diameter, and thread pitch if frictionless. The K factor introduces two extra terms, one for the thread friction and one for the washer friction. The Clamping force of a torque is reduced by the friction. Adding grease cuts the friction ~ in half.

I am suggesting that an action, like the Rem700, that only stops at the large ring, and thus only puts barrel threads in tension near the front, requires enough clamping force such that it is greater than the force from firing a round. That clamping force requires less torque with grease, but is the same clamping force as dry.

The notion that barrel threads suffer from repetitive stress fatigue failure is not one born out by my experience, but on an academic level, it is my contention that if the torque is reduced on greased threads the same accuracy integrity [not shoot loose] could be maintained at the same thread stress as with dry threads.

What does it all mean?
You can use grease, and not need as much torque.
Why did you buy a Rem700, instead of a Mauser with inner C ring to also compress barrel threads at the breech?