Random topic I'm thinking about while battling insomnia. How can tightening a flash hider cause an index pin to shear if a barrel is already torqued in a receiver with a gas tube installed? I'm thinking the torque applied would have to be greater than the torque applied to the barrel nut plus the torque required to shear the pin. Also with the threads Ive seen on this, how would anyone see if the index pin was sheared since the barrel but assembly covers the pin visually.. GD fill me with knowledge.

Using a crush washer? I doubt you could.

But still it's always best to hold the rifle by the barrel.

You'll know because -

1. The feed ramps on your barrel extension won't be lined up.  Your BCG likely won't go into battery.  
2. Your gas tube will be tweaked/bent/crimped.  Some people have spun their barrel 1/2 revolution or more before realizing what happened.  
3. Or, worst case scenario, the index pin won't shear but (because of improper torquing from the factory) your barrel will turn in your barrel extension; resulting in replacement or very costly repairs compared to a sheared index pin.  And yes, this scenario has happened to members here.  Improper procedure in the meloniting process can significantly decrease the torque spec on the barrel extension.  

Bottom line, your best bet is to clamp the barrel when installing a muzzle device.  Yes, 99% of the time everything will be OK, but it only takes once to make for a poopy day.  
In theory, it should never happen; but obviously that is not the case.  A couple of chunks of 2X4 and a bench vise is cheap insurance.

I understand when and where to use a padded vise and all but that's always good information for someone to read. I'm having a hard time seeing how a muzzle device could rotate a barrel when the barrel nut is torqued to 30-80 ft-lbs. I don't recall a crush washer ever taking anywhere near that on any of my builds.