I understand the Ph thoughts on a preference for certain calibers. They see alot of shooting by alot of shooters with varied ability. I prefer, if it is a PG hunt, to go with the thought of using a caliber you are familar with along with a good bullet. I think most deer hunters have a 308, 270, or 30/06. I think deer hunters and especially western deer hunters are perfect prey for finding a desire to chase the "lopes" Africa has to offer.
When traveling with a group it may be easie as mentioned to borrow even a buddies rifle especially if it is 2x1 hunt. If you think you need more.
I would take the 270 wsm if he shoots it well. I would be more inclined to suggest taking this rifle and invest in the cost of ammo for practice before the trip. I for one was not a natural for shooting off of standing sticks. I do alot of shooting with rimfires off of sticks as well now to get better. The practice part does not get mentioned enough in African shooting. The way it is done may or may not allow time for a rest from a normal shooting position as you would in the states. Standing shooting off of sticks to see over the brush is a common method in Africa. No two places in Africa are the same. Ask about the distance you will encounter as a normal distance. Practice now and win over the PH when you get to the bush.
I will probably be taking a 270 of some type this summer myself. I will share it with my oldest son. I am leaning on the 270 WSM as I would like to get some more time in the field with it. I have barrels in both calibers so it is just up to me to decide. Neither are bad choices.
