The option that you mention would be the one I would use if I could find a suitable barrel at a reasonable price. That's why i was looking at the Mosin barrels , 30 inches long and 311 - 312 bore, the problem is getting it to Australia.

www.ebay.com/itm/Finnish-M39-Mosin-Nagant-Surplus-Heavy-Barrel-With-Sights-7-62x54r-1942-SAKO-/372231104298