Have you ever heard of bending an Upper receiver?

[NugginFutz]

Correct - with the reaction rod, the BARREL is out of the torque path. But the same sheering forces will be applied to the PIN AND SLOT whichever end you secure the assembly with.

Don't believe me? Get a 4" diameter length of PVC and cut a hole in it near the end. Slide your forearm into the tube and extend your thumb through the hole. Next have someone hold your exposed hand, which is protruding out the end where your thumb is extended through the hole. Now have them twist the tube until your thumb hurts. Stop. Now have them let go of your hand, and twist the tube again. Stop when your thumb hurts again. That was your elbow and shoulder preventing your thumb from rotating with the tube.

The handshake represents the reaction rod. Your elbow and shoulder represent the barrel clamp. Either way you do it, your thumb gets to feel some pain.

[Wheelhorse]

Correct - with the reaction rod, the BARREL is out of the torque path. But the same sheering forces will be applied to the PIN AND SLOT whichever end you secure the assembly with.

Don't believe me? Get a 4" diameter length of PVC and cut a hole in it near the end. Slide your forearm into the tube and extend your thumb through the hole. Next have someone hold your exposed hand, which is protruding out the end where your thumb is extended through the hole. Now have them twist the tube until your thumb hurts. Stop. Now have them let go of your hand, and twist the tube again. Stop when your thumb hurts again. That was your elbow and shoulder preventing your thumb from rotating with the tube.

The handshake represents the reaction rod. Your elbow and shoulder represent the barrel clamp. Either way you do it, your thumb gets to feel some pain.

I agree except for one thing. Your test doesn't take into account the lessening of the twisting force on the pin as the barrel nut is tightened. The tighter it gets, the less stress there is between the protruding pin and the slot in the receiver as everything is squeezed together. Remember, we are turning the barrel nut, not the barrel extension and with a little grease or anti seize applied, the rotational force transmitted to the extension is going to be minimal.
I still think that the most direct, with the least number of parts involved way to counteract the force of turning a barrel nut, is with a tool that locks in the lugs of the barrel extension.
Thats my story and I'm stickin to it.

[montana]

I have to "respectfully" disagree . The barrel extension tool is holding the barrel extension which is screwed to the barrel which stresses the barrel from the rear rather than the front of the chamber. The stresses will be the same to the upper receiver no matter if the barrel is being held stationary from the front or rear of the chamber. The lessening of the twisting forces on the pin as the barrel nut is tightened would be same no matter which method is used. If this is not the case someone with a higher pay scale than me would need to explain it.

[Keep The Change]

Been a long time since I've been on here....just haven't had much time to focus on any hobbies with work and kiddos. But let me jump in here on this topic.

I think the Reaction Rod is a very good way to go for installing and removing barrels.
The rod interlocks with the barrel extension, thus any torque applied by removing or installing a barrel nut is all reacted at the interface of the barrel extension and reaction rod. No torque will go through the barrel or index pin.
As you apply torque on barrel nut the upper wants to act as a single piece, the reaction rod grips the extension and counteracts the torque. Torque only comes out at rod interface and doesn't affect barrel at all. Now if you turned the reaction rod without reacting it at the barrel nut you would load up the index pin and not the barrel in that case either.

Take the clam shell design for vices. The torque is applied to the barrel nut and the upper wants to act as a unit and spin until something counters it. In this case the vice reacts the torque at the top and bottom of the upper.

[Keep The Change]

Now lets discuss stresses in the upper if you are using a device that clamps on the upper, thus causing all the torque forces to be reacted by the upper.

Torque on barrel nut is reacted by the threads, index pin, and friction, which ultimately is reacted by the cylinder section of the upper. It creates a torsion and shear force through the cylinder section of the upper. The torque value of 80 ft-lb on the nut creates about 4000 psi of shear stress in the cylinder of the upper. The shear allowable for 7075 Forging is 44,000 psi. No big deal right?

Well as that torsion load travels back down the upper, it comes to some cross sections that are not continuous like the cylinder portion of the upper.
The upper transitions from a continuous cylinder into a more rectangular section that is not continuous at the bottom and even at the ejection port.

So what are the shear stress values now at these sections?

For 80 ft-lb torque a continuous rectangular tube approximately the size of the upper still generates about 4800 psi of shear stress.

For 80 ft-lb torque reacted by a channel (3 sided square tube) of the same thickness, the shear stress goes up almost 10 fold, at around 48000 psi above the allowable for the material.

So if you clamped an upper from the middle of the ejection port and back, you could really do some mangling on an upper. Because the full torque will have to be reacted right were the clamping begins.

I did this study just as a reminder of how important it is to realize how easy it could be to distort an upper if you aren't careful about thinking where this torque load is going to be reacted.

Imagine clamping locally, just the behind the barrel nut threads. That would be much better than clamping just behind the ejection port, right? Because the barrel nut thread area is continuous, whereas around the ejection port area has 2 locations that aren't continuous.

That is why I think the reaction rod is a great idea and tool and I think I'm going to get one. It keeps all torque load out of the upper, index pin, and barrel. It is only reacted at the barrel extension lugs.

The clamshells and the cutting board types (which is what I currently have) that LRRPF52 suggested, are fine if you are installing a barrel at the ranges specified by the armorer spec, which I believe is between 50-80 ft-lbs. But if you are a removing a loctited barrel, I would use the reaction rod type.

[Variable]

Well, since everyone seems to be chiming in.LOL

Heres my opinion:

A barrel nut screws on to the receiver. Period. If you lube the receiver threads and the front of the barrel extension, almost every single drop of torque is directly between the barrel nut and the receiver threads. Using a clamshell or cutting boards keeps it there. I use a clamshell with an anti-crush insert.

When you use a "reaction rod" you are no longer supporting the receiver at all. You are fixing the barrel extension in place. The barrel extension isn't receiving the barrel nut, the receiver is... When you torque the barrel nut onto the receiver threads, the receiver tries to turn (communicating the torque on down the line). The only thing resisting that torque is the barrel extension's index pin. Otherwise your receiver would simply spin in circles.

Using the other Brownell's rod (while fixing the barrel nut wrench in a vise) does the exact same thing in the opposite direction. The rod turns the barrel extension. The barrel extension would then simply spin forever unless that rotational torque is then transmitted to the receiver by the index pin. The index pin spins the receiver, and the receiver (where the threads are--- not on the barrel extension) gets torqued into the barrel nut.

Whichever rod you use, it transmits torque through the receiver via the barrel extension index pin. There's absolutely no way around that.

[NugginFutz]

Coincidence? Perhaps. Relevant? Absolutely!

http://www.ar15.com/forums/t_3_4/615551_Reaction_Rod_question_issue.html

June, of 2013.

[Variable]

The tool Windham uses in the link posted on that page ( http://www.ar15.com/forums/t_2_489/2...age=1#i2027548 ) brought a tear of joy to my eye.



Holy crap, I'd like to have one of those! I'd rather not pay $400 for it, but I'd pay $200 without blinking. That is pure beauty.

[NugginFutz]

Yeah - I saw that, too. It really does address all the shortcomings of either the receiver blocks or the barrel blocks/reaction rods.

I read where he was considering, albeit briefly, of going into production. Makes me wish I had a machine shop at my disposal.

[Variable]

Yeah - I saw that, too. It really does address all the shortcomings of either the receiver blocks or the barrel blocks/reaction rods.

I read where he was considering, albeit briefly, of going into production. Makes me wish I hade a machine shop at my disposal.

LOL
I fired off a pic and some links to the guy that made my Al barrel nut over on Arf. He's already modded his own Reaction Rod with a pin to retain the receiver on the rod. Maybe he might be interested...

[NugginFutz]

That pin, by the way, is also used to help align the barrel nut. No?

[Variable]

That pin, by the way, is also used to help align the barrel nut. No?

Yep. They are using a bent gas tube shoved through a hole drilled through the charging handle channel's indexing spline. It'd make assembly as easy as can possibly be. I saw that and slapped my head. It's truly an awesome tool IMHO.

Just keep a tiny bit of forward tension against that tube, and as soon as the barrel nut lines up enough it'd pop through and tell you you were home.

That tool would make upper receiver building a "point, set, and match".

[Keep The Change]

Well, since everyone seems to be chiming in.LOL

Heres my opinion:

A barrel nut screws on to the receiver. Period. If you lube the receiver threads and the front of the barrel extension, almost every single drop of torque is directly between the barrel nut and the receiver threads. Using a clamshell or cutting boards keeps it there. I use a clamshell with an anti-crush insert.

When you use a "reaction rod" you are no longer supporting the receiver at all. You are fixing the barrel extension in place. The barrel extension isn't receiving the barrel nut, the receiver is... When you torque the barrel nut onto the receiver threads, the receiver tries to turn (communicating the torque on down the line). The only thing resisting that torque is the barrel extension's index pin. Otherwise your receiver would simply spin in circles.

Using the other Brownell's rod (while fixing the barrel nut wrench in a vise) does the exact same thing in the opposite direction. The rod turns the barrel extension. The barrel extension would then simply spin forever unless that rotational torque is then transmitted to the receiver by the index pin. The index pin spins the receiver, and the receiver (where the threads are--- not on the barrel extension) gets torqued into the barrel nut.

Whichever rod you use, it transmits torque through the receiver via the barrel extension index pin. There's absolutely no way around that.

You are correct, I stand corrected.
The index pin will still be loading up the receiver when using the reaction rod. I was ignoring the torque that will still go into the receiver due to friction at the threads.


The torqueing of the nut creates friction at the threads and at the flange of extension/barrel nut. Also the turning of the nut puts tension in the receiver at the threads, creating more and more friction as torque load increases.

Using clamshell type or cutting board config:
-The receiver wants to rotate due to friction in threads. It is prevented by the vice/clamping device and the torque is absorbed in receiver cylinder. The index pin will contact the upper due to friction load created at the extension flange & nut interfaces. That friction will cause the extension to rotate and put the pin in contact with the upper.

*The receiver cylinder where it is clamped by the vice reacts the torque generated by the thread friction.
*The index pin contacting the receiver is reacting the torque created by friction btwn extension flange and barrel nut.
*Torques created by 2 notes above, create torsion in the upper at the location it is clamped.


Using just the reaction rod:
-The upper is rotated when the torqueing begins due to same friction at threads. It is rotated till it bumps into index pin.

*The index pin is now reacting the torque created by the thread friction.
*The reaction rod is now reacting the torque created by the extension flange & barrel nut friction. This is the only thing it can react.
*Now the upper is only reacting the torque created by the thread friction at the index pin.

So the reaction rod helps reduce the torsion the upper has to take, but it still takes torsion at the index pin contact.

Thanks for correcting me Variable. I didn't think it through completely.

[Variable]

You nailed it with an even better description than I did.

I thought the other way at first too. I had to sit and ponder on it for a while to see where it actually went in my riddled brain.LOL

[Keep The Change]

Yes it's a bit confusing. I had to sit and visualize this for awhile since I don't really want to take one of mine apart to prove it to myself.

The Reaction Rod does provide some benefit, but I think the clamping method works just fine IMO.