I agree with gryphon that the most important aspect of a SAmbar cartridge is that you shoot it well. so if that means 3006 as appossed to 338 win mag then so be it.
The last and my only sambar was shot with a 3030. I was not impressed with the killing power even though I shot it in the spine just behind the shoulder blades as it walked in the gully below me. I have a 7mm deer rifle now and although I shoot a few reds and fallows I am researching for a Sambar rifle. Sambar deer don't give up the way red's just seem to fall over and lay there. They keep kicking and going even if this is only the front half. The 3030 bullet did not exit the spine. If I had missed by an inch it would have still died as the bullet would have hit the lungs but tracking in that thick country would have been a real issue.
My 2 cents is you have already fired the 338. If it wasn't unpleasant buy it for the RIGHT price then after practicing offhand with a hundred or so rounds it should be fine. When recommending a pure sambar round I think 35 whelan and up. If you finally cross paths with a Sambar you want an exit hole and preferably it to drop on the spot. You already have a deer cartridge may as well take a real step up.