You use the gun that the guides provide you,you shoot only when they tell you to shoot and at what elephant they tell you to shoot .
And you make sure that there is a qualified guide that is a marksman with you just in case you make Mr.Elephant mad by bouncing a large caliber rifle round off his thick skull instead of bashing it into it.
I have not heard of PH's ('professional hunters', rather than 'guides') supplying a rifle to use as the norm, unless the clients gun malfunctions/breaks during the trip. First of all, most PHs use pretty spendy guns (its their living and life on the line) and, more importantly, they expect their client to be able to actually use the rifle proficiently, which means a lot of practice with the gun you are going to be using BEFORE you show up in the bush.
As far as shooting when they tell you, that is true.
Secondly, the only way to legally hunt in Africa (so far as I am aware) for a foreigner is with a Professional Hunter, and while their are some better than others, if they have been in the business very long, you can bet they know how to put an animal down in a hurry.
Again, to shop for a rifle, buy the book 'Dangerous-Game Rifles' by Terry Wieland.
The author has been on dozens of safaris, and has the money and/or connections to have used dozens of different guns.
The book covers doubles and bolt actions, different manufacturers, cartridges, and a section on practicing for a safari.
Eli W.