Get tough.
Apply some Kroil to the drum and let soak 24 hours.
Pick a socket from a socket set that will fit over the sight drum.
Put some sheet brass over the other side of the drum to protect the finish.
Put it in a vise making sure the socket is over the drum and the brass is protecting the other side and tighten the vise to press the drum over.
If the drum requires so much movement that the drum is level with the sight base, put a socket with a small end that's smaller than the drum on that side.
