What I do is take a piece of rifle barrel and turn it so that it looks like a cartridge case, and fits the chamber. It must project back from the barrel to reach the bolt face, taking up the space occupied by the .303 rim. Or, the chamber in the barrel could be drilled and bored out, and a cylindrical insert made. Six of one... Once the chamber insert is bonded in place, the liner drill is run through, and the liner is bonded in place. To convert the bolt head, I annealed it, and drilled through for a 1/4" diameter plug, which I soldered in place. Carefully lay out the position of the rimfire firing pin, drill a two diameter hole, and make a firing pin to fit. Shorten the original firing pin, so that it strikes the new firing pin.
Converted a No. 4 a while back, simulating a No. 9 rifle.