I would definitely defer to Gryphon on any advice on sambar.

But if you can handle a .338 go for it. If you can shoot one first try it. There is no reason a .308 or .30-06 won't do the job though. When I first shot my .30-06 I didn't like the recoil. Recoil tolerance is an accumulative experience for many people. Shoot it but not from a bench and see what your 'next' calibre is like.

A .338 has to be one of the most versatile calibres for hunting almost anything around the world. And is a good addition to your 6.5mm.

Good luck on the fallow's next season too.