In the U.S. this is do-able

Krieger has a .330 bbls.

You could make it a 318 WR-Flanged using .400-350 brass.

Your bullet selection is limited.

Any reamer maker can set you up with a reamer.

I can only see one reason for doing this cartridge and that is nostalgia. If I didn't think that was worth something I would shoot nothing but Remingchesters in their new .281 ultralong short semi belted magnum instead building Martini's in 450 Musket No.2.

If I were to do this I would use a .338 caliber barrel. On the top I would mark the caliber .318 Westley Richards Flanged. On the bottom I would mark the barrel .330 bore/.338 groove.

While trying to find information on this cartridge I was struck by its similarities to the 338/06. Just minor differences. I have a 338/06 AI on a Mauser that I built years ago at school. I have taken more large game with that gun than any other rifle I own. For years my favorite bullet for the gun was the Sierra 250 gr. SPBT. At 338/06 velocities I got none of the problems of blow-ups that are associated with this bullet in the .338 Magnum. Several years ago I switched to 180 grain Nosler Ballistic tips and found them a perfect match to the cartridge. Their extreme accuracy is especially helpful at Montana's longer ranges. They have good killing power and controlled expansion at the lower velocities associated with this caliber. I have used the bullets on White tail and Mule deer, and have been very satisfied.

I used the .200 gr Ballistic tip in my 338/308 at 2600fps on Elk and Plains game on my Africa Trip, superb bullet in this use. Maybe I should try them in a 318 WR-Flanged

John shame on you. Now you got me thinking. I need to locate a Martini action so I can build a .318 WR-F. Swell just what I need, another project! Oh well I retire in 2 years 8 months, so I will need something to keep me busy.