Classic, classic, classic... and sad... but easily fixable...
And therein lays the demonstration, if any was ever needed, that:
1) Rifles in this power class should weigh 10.5 to 11 lbs. for most of us mere mortals, which is what they have been made to weigh, and for good reason, for close to 100 years.
2) Rifles in this power class should be shot from the old British
standing bench.
So, how do we fix this? Quite simple:
1) I would advise against anything mercury these days - these are technically not allowed on airplanes anymore, but reducers that add dynamic effect to mass are more effective that simple lead weights.
Consider installing a Blaser 16 ounce tungsten powder "kickstop" in the stock. It will bring the rifle to 10 lbs. 9 oz. where it should have been to begin with, and will significantly reduce felt recoil. Any reasonably careful man can do this, but just to be on the safe side, have a qualified stock-smith do it to avoid adding insult to injury
2) Only glutton-for-punishment folks shoot rifles in this power class from the classic American bench. At the very least, shoot from a high table and low stool so that the back is straight and allowed to roll back with recoil, as opposed to hunched down on the stock, or better,
consider shooting standing from the (tripod) sticks to allow the entire body to roll back with recoil. Good technique on the sticks will provide close to benchrest steadiness, and in any case enough steadiness for 50 yards open sights shooting.
3) Consider trying the above #1 and #2 before selling the rifle, while keeping in mind that many/most golden age and modern PHs who made a name hunting elephants (among the modern ones, Buzz Charlton comes to mind because I just read his book and saw his videos where he shows quite convincingly that a .40 is a great killer but a poor stopper; or Wayne Van Den Bergh of
Nyamazana Safaris with whom I will be hunting elephant this coming August and who swears by his .458 Lott) will tell you that
the 450-400 is a great caliber but it is no .450 / .470 / .500 when the chips are down and an infuriated "Zambesi Lady" elephant cow is coming...
Rare Breed, dear friend, do not walk away yet, your trouble is easy to fix

and you are not the first one learning to shoot a double either
