What is your vise jaw padding of choice to hold the lower receiver secure for...
removal/installation of the receiver extension?
What can withstand the 40 ft-lbs of torque without to much flexing?
You'd think that by now they would have come out with something like the upper receiver clam shell but for the lower receiver instead.
What is your vise jaw padding of choice to hold the lower receiver secure for...
Several companies make lower receiver vise blocks. Simply stick it in the magwell and into the vise. Here's the Brownells for example.
http://img.tapatalk.com/d/14/03/26/esy9aseq.jpg