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That's why i was looking for something different than my dpms claw. Had a tough barrel nut to remove and the claw worked but barely. The whole fixture that wasn't in the vice seemed to move.
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Yes, exactly... The DPMS Panther Claw vise block can become flexible on tight/stuck barrel nut removals, and it's possible to crack the upper at the lugs with it. Though it's usually okay to install barrel nuts with it due to it's internal support.
My procedure for removing barrel nuts involves inserting the 2UniqueLLC barreling jig into the upper receiver/barrel extension for internal support, then I clamp the upper receiver in the vise with a set of clam shell upper receiver vise blocks. You could also do this for installing barrel nuts if you want to be overly cautious.