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Kit includes everything you need, except a piece of 1/4 20 threaded rod.

Mount the fixture to the trunion using the two outer bolts, (these only need to be snug).

Put the pushing plug in the end of the barrel and tighten the center bolt.

Hold and turn the center bolt. You can use a vise, but clamping it to the bench works too. ( did you remove the barrel pin?)

Reinstalling the barrel

Start the barrel by hand. remove the muzzle break. Insert the threaded rod bushing and nut ( I wrap the rod with tape to protect the bore).

On the other end insert the trunion adaptor and other nut.

As you're tightening the rod, you my have to tap as shown, most of the time you don't.