Must Have M4 Specialty Assembly Tools

[lysander]

The relation between the gas block and receiver doesnt change when a block flexes.

But, the rotation of the nut does, flex in the fixture make aligning the nut to the hole "harder than it needs to be."

[lysander]

No. The barrel will never rotate in a block because you are torquing the barrel nut onto the upper that is clamped.

With a receiver block there is no need to prevent the barrel from rotating. It sits there in the upper indexed with a pin. There is NO rotational torque being applied to it. The barrel nut is being torque to the upper. Not the barrel.

Your knowledge of physics is lacking.....

The nut comes in contact with the flange on the barrel, friction between the nut and the barrel flange imparts rotation on the barrel, see post #9.

If you doubt this, take the indexing pin out of a barrel assembly, put an upper in some blocks and torque a nut down on it. The barrel will turn as to start to get to 30 ft-lbs.

[MorphCross]

There is a virtually nonexistent likelihood of the Super Reaction Rod, the Midwest Ind URR, or even the Magpul Bev Block allowing the index pin to overcome the square opening in the upper receiver. All three of those utilize some method of locking the upper against the Rod or the block along with the barrel extension splines.

That being said I am still the fuddy-duddy luddite that clamps a barrel close to the muzzle device in leather padded rosin powdered blocks for installing muzzle devices that specifically mention torque values in their installation.

[AndyLate]

This is pretty obvious until it isn't - pistol caliber and rimfire builds require receiver or barrel blocks. I am spoiled by using my reaction rod, and felt like a caveman using my receiver blocks assembling a 9mm upper.

Andy

[Rifleman_04]

.....

[MistWolf]

And, if the upper is held in blocks, the barrel tries to rotate with the nut, loading the indexing pin against the slot in the upper.

Like you said - six of one half dozen of the other.

Exactly

[MistWolf]

I haven't exceeded 50 ft lbs installing a barrel nut. Shouldn't be a problem I wouldn't think.

You're right. The point is, if you use a receiver block, the barrel will turn and the index pin will press against the slot in the receiver. If you use a Reaction Rod, the receiver will turn and press the slot against the index pin.

The relation between the gas block and receiver doesnt change when a block flexes.

Quite true. However, torque that should be used to move the barrel nut is wasted by flexing the block instead. That means more torque has to be applied to get the nut to move. Once the nut starts moving, the peak amount of torque needed is reduced. However, excessive torque has been loaded into the block. The block springs back, sending the excess torque back to the nut increasing the amount of torque applied and moves the nut more than desired. Now, a back and forth game is played while trying to get the gas tube notch lined up just so. Alignment is particularly important when working on a Colt because the hole through the upper is smaller than some economy brands.

If the receiver block were made out of steel or aluminum, it would be a much better tool.

[MistWolf]

Well I'm a Marine. Not a physicist. Guess I'll just keep my real world experience of maintaining and fixing M16 FOW guns for Mil/LE to myself.

Your real world experience is very important and is to be respected, but all of us can learn from the experience and knowledge of others. Although I've learned much from working on ARs over the years, my experience doesn't match that of those like you who work on ARs daily. But I also have a perspective on technical details gained from more than thirty years of working in aviation. I don't say any of this trumps what you know- far from it. But I do notice things like tooling fixtures (such as a plastic upper receiver block) having flaws and how to either fix or work around those flaws.

Trust me- sharing your experiences will help us all to learn. I know you have tips and tricks I'm not aware exist. I gained a wealth of knowledge taking an armorer's class from Will Larson and I still feel like a raw beginner

[titsonritz]

I wouldn't consider the fitting pin a "must have". It would be nice to have for popping pins out, but useless for installations. By the time I got the holes lined up to insert the fitting pin, I might as well as insert the trigger/hammer pin.

When it comes to installing a trigger, my "must have" tool is a slave pin for the trigger disconnect.

This. I use my pin punches as slave pins for everything else, but for the trigger/disconnector a shorty slave pin that fits in the receiver is where it's at.

[26 Inf]

I have a lot of AR tools, but this one has definitely made life easier, for me anyways:

https://www.knsprecisioninc.com/product/hammer-cage/