Best way to clamp a tapered barrel on a rifle

[Ura-Ki]

I use two pieces of Channel Iron filled with strips of lead that I shaped rough to the barrel, then clamp them in a big bench vice and tighten down on the barrel enough to form the lead around the barrel nice and tight! It will not slip, or mar a finish, and it will hold tight no matter how stuck your barrel is!
Conversely, A couple pieces of good hard wood shaped to the barrel and lined with powdered resin ( Pitch dried out and powdered works well) and same deal, clamp it nice and tight in a bench vice and get to work!

 

[Steel Head]

Put a wrench on the brake and smack the end of the wrench with a hammer or even better a dead blow.
you shouldn't need a barrel vice to install/uninstall a brake.