After a decade plus of assembling uppers here's where I've arrived:
The clear majority of the torque is between the barrel nut and the upper receiver... I've seen it visually, and the physics makes sense as well.
Therefore it makes sense to clamp the upper receiver as the wrench is on the barrel nut.
This means one of many options:
- the traditional clamshell, which most of us tend to not like. And yet most of the really seasoned hands say it's still one of the best solutions for mil-spec sized receivers. IE: not billet or other oddities.
- The DPMS type nylon block that holds the upper receiver through the pivot pin holes. I know individuals who assemble a lot of uppers commercially and this is all they use.
If you know how to torque and end up on the lower end of GI spec by following the procedure or by using shims, this works well. I have one and it's pretty secure even though I question the stress on the pivot pin holes.
I would not use it for untorquing unknown barrel nuts that might be significantly over torqued or have loctite.
- The Bev block is an interesting compromise, and does the job. But allows more flex than what I would prefer. Which you could say "how does the Bev block flex as it locks both"? The issue is that the barrel does not move much, the receiver does. And there is enough slack in the fit to the receiver in most cases that it allows just a little bit of flex.
The MI tool might very well be the best of all worlds.
But based on my experiences I'm kind of wishing I just bought a good clamshell way back. Though I couldn't have used it on billet (sun devil) receivers, or it's more difficult.
Reaction rods and similar are useful for other things. I know all people using them in production environments due to speed, and I sort of understand that.
The bev block is very useful for holding receivers for work, etc in addition to torquing barrel nuts. It's a bit more fiddly, so I could see folks preferring other methods for production.
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