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John, I bet they had some lock-tite on the original install.
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I've heard of at least one manufacturer doing that, but their barrel nuts supposedly require heating with a torch, before they will let go.
The assembly book that I bought when I built my first AR, suggested putting a little lube on the threads, since there is a chance of the dry threads seizing over time (dissimilar metals). Dry threads, plus having to turn the nut a bit more than normal to get the gas tube to clear, would probably be a royal pain to break loose.
I'm not far from Bellevue, but my vice is currently in my Father's basement. I've assembled uppers with the little 4 inch vice on my toolbox at work, so you don't need much of a vice for assembly.
If it's raining this weekend, that will kill the majority of my plans for the weekend. Even if it's not raining, driving to Bellevue and assembling an upper would be more fun that most of what's planned for my weekend.