Firstly, I'd never claim "vast" - all my experience is as a client who's success has come from always following those with the vast experience - my PHs. But what they always tell me is that (as general rule) as long as you can constantly bounce a mug around at 50yds with your 458 that's good enough - so a 2-3" group should see you ok. Remember too, the heart on a cape buffalo is about the size of a grapefruit, and the closer you are to him (and preferably unaware of you), the easier it is to hit it. So for any situation, I'd also advise knowing the POI for your ammunition at ranges from 25 to 100yds: for myself I like to get in around 40-50yds before opening the proceedings.
Placement of that all-important shot is everything, and is dependent on the angle and anatomical landmarks presented to you. So at this moment follow your PH's whispered advice. Using this, while getting on the sticks and sighting in, I'm thinking things a bit further by using those anatomical landmarks on the skin as a guide while visualising I've got a long knitting needle with which to pierce the heart (or whichever of the vitals available). Then placing my sights where the "needle" intersects the animal's skin I squeeze off the shot.
Remember too, that as soon as you've fired - RELOAD!! - and get ready for the follow-up. DON'T stop there to admire your shot!! That's a very good way to lose your buff, and greatly annoy your PH - as he's the one that has to now go get it, AND keep yours and everyone else's butt intact! Remember, there'll be plenty of time to admire your buff later, once it IS dead.
So for practise at the range before you depart on your hunt, train yourself into the habit in taking that first, steady, shot off the sticks and then - as you come out of recoil - work the bolt to chamber another round and get back on target asap. Generally, after the first shot there's far too little time to get back onto the sticks, so your second shot may well have to be freehand.
As a reloader, I've developed a DG load for my rifle and found it is both accurate and performs beautifully. A lot of PHs like a soft on the first round, followed up by solids, but I've hunted with others that like you loaded up with softs only (this prevents "shoot through" if another bull is behind yours), so check with your PH first.
The photo below is clipped footage of one of my Zambian buff, and underscores much of the foregoing. That's it's heart being held in the trackers hands (it's apex, or bottom, pointing towards his feet), and the large hole you can see is the exit wound of a Woodleigh 500ger RNSN projectile. That buff wobbled about 20yds before collapsing, and it was all over.
Hope this helps, and enjoy your journey!!
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