What Big Duke says is absolutely correct. My friend has a 7.62mm No5 rifle with a barrel machined from a new chrome lined 7,62mm L4 barrel. He ain't in the winning prizes league but just good enjoyable shooting. A lot of the target rifle barrels have so much meat on them that it seems a shame NOT to machine one down to fit your No4. Don't faff around with all that foresight block and bayonet lug palava. Just bore out the muzzle end of your shot-out .303 barrel back to the rear end of the foresight block band lugs, accurately cut off. Machine down your newly threaded and profiled ''7.62mm No4' barrel to suit........., tight fit....., sweat both surfaces, slide on........, accurately align........, allow to cool........, make good and it's there forever. Done it about 5 times now!

The GOOD thing is that under the fore-end you can have a reasonably heavy barrel