Thanks to everyone who responded.

I live up in the sticks, ,and I don't think that there is a side-by-side double barreled shotgun in town - only pumps and auto's, and a few single triggered over-unders.

The comments as to trigger control are interesting, and apparently I was thinking incorrectly. I would of thought rear trigger first because this is the trigger which allows the most natural hand position, and therefore provides the best advantage for an initial shot, then as the rifle recoils the hand slides forward on the pistol grip, and is in position to engage the front trigger on follow up. Yes, I have trained myself to shoot both a rifle and a handgun with just the front pad of the finger, so using full finger contact on the trigger might take some getting used to. With regards to the foerend - Yes I will grip the barrels, provided the forend is suitably shaped.

I don't think recoil will be an issue. I've owned a number of light for caliber rifles, and I could hold any of them for 3 shots from prone. Agreed, a .458 is not a .500, but then, neither do I anticipate shooting the .500 from prone.

The ammunition is being bought by the owner of the rifle, and my only input is to pay for it. I've ordered 50 rounds, and I hope this will be enough to familiarize myself to a new rifle and technique with some left over for hunting.