Six and a half pounds? Ouch! My +10 lb 404J on 98 Mauser is about all I can handle ... IF I hang onto it FIRMLY. I suggest building something with more heft and less recoil. I carry that rifle all day in Africa no problem.
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And when upland hunting back home in Montana I carry a heavy old Browning A5 Magnum Twelve all day essentially every day for six weeks every year. And I'm no Hulk Hogan.
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Honestly, I do not understand the current trend in lightweight guns. Why would anyone want a lightweight 3.5" twelve gauge magnum to hunt waterfowl? Generally waterfowl hunters don't need to carry their guns any further than the truck/boat to the blind. A lightweight dangerous game rifle makes about as much sense to me = very little.
A word of caution about delaying your safari to "bucket list." You don't know what the future will hold re your physical capabilities, especially in the golden years. I see way too many guys waiting until their health is in the toilet and then abandon the idea or make compromises that tend to significantly depreciate the experience. I just finished my fifth safari and I will be 74 in a few weeks. I also live in an 800 sq ft home built in 1929 and I drive a 2004 vehicle with 200K miles. For a long time I have had my priorities geared to hunting wherever I can as long as I can. Check it out. I think you will find a quality safari is not as expensive as you might think. Don't discount South Africa. I hunt an Eastern Cape property that's 167K acres and most of it is inaccessible. There are fences but I rarely if ever see them. And I guess you didn't know you can't take your guns with you to Zimbabwe (or so I have read)? You might enjoy hunting Mozambique ... if you enjoy living on the edge ... and can afford your own private army security detail. Namibia by all accounts would also be an excellent choice, especially for plains game. And finally, don't overrate Cape buffalo. I have shot three and the experience can be very challenging. But the pricetag is also very challenging. I find kudu are just as challenging to hunt and probably the world's most beautiful trophy. You could shoot five kudu (maybe more!) for the price of one Cape buffalo bull.