Your concern is legitimate. You should prepare for every possible contingency.

Discuss your concern with your PH. Can he get ammo in your caliber? Will he have a back up gun and ammo if your rifle or ammo doesn't arrive?

Now if you are absolutely over whelmed with concern and you are adamant that you must only use your own gun then you should only carry a gun and ammo chambered in .308 Winchester, .303 British and 7.62 x 39.

It's almost a guarantee that if your ammo doesn't arrive you will be able to find ammo in these three calibers. If your ammo arrives and your rifle doesn't you are more likely to find a rifle in one of these three calibers than any others.