Oh that's easy - it will simply be chambered in a cartridge that Mr. Fraser would have used! A superlative rifle in the Scottish style should only be chambered as such! Here are the options I am presently entertaining for single shots:

1. .400/.360
Perhaps nothing says "Fraser" more than this cartridge. Something marked "Velox" would be on equal footing, but those were usually reserved for Fraser's magazine rifles (.303 Velox, .256 Velox, etc.)

2. .303

3. .256 Dutch Mannlicher

Now, we intend to alter these actions to be take-downs duplicating Fraser's method (Release lever on left side of action, screw in barrel, and forend with Deeley latch). As such, a cased multi-barrel set could easily be made for a client. In which case, I think options 1 and 3 make sense. As a single barrel, option 2 or 3 get my pick as they're both so versatile and pleasant shooting.