I’m pretty sure Krieghoff does custom calibre guns in just about whatever you can think of though they charge extra for it obviously. Their Optima is also the only drilling I’ve seen in nitro chambering. They also make other triples in smaller calibers and with normal gun weight. A triple gun I thought would be great for NSW public land hunting is Blaser BD14. For rifled barrels I’d go with 222 or 5.6x52R for small barrel and 30R Blaser or 9.3x74R for the large barrel.
I'm on another forum where a member recently acquired a BD14 and says he loves it! The basic options from Krieghoff are as follows...(they have a very useful publicly available full price list .pdf)
Krieghoffs drilling rifle barrel(s):
6x70R, 6.5x57R, 6.5x65R, 7x57R, 7x65R, .308 Win, .30-06, .30R Blaser, 8.5x63R, 8x57JRS, 9.3x74R & (.30-06, .270, .243 Win., .222 Rem at additional costs)
Krieghoff can factory fit their own rifle barrel inserts into shotgun barrels in newly purchased Drillings:
.22 lr, .22 mag .22 Hornet, .222 Rem., 5.6x50R Mag., 5.6x52R Savage, 6x70R, 7x57R, 7x65R, .30R Blaser, 8x57JRS, 8x75RS, .308 Win., .30-06, 9.3x74R.
So with the above choices in mind & some chatting with mates to reconsider things I've changed to... .222 Rem, 6.5x57R, .30-06, since I'm capable of getting my desires from those calibers in both hunting & rifle designs.