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While I readily admit that the .300 RUM is a more modern design, it still is but a ballistic twin of the .300 Wby and I would speculate that .300 Wby that have been in the market for 70 years for tens' of thousands of .300 Wby rifles will still be with us as long as hunting exists.
I am not sure how long .300 RUM ammo will be available. I doubt enough rifles have been sold to justify corporate America to keep loading for it.
Also, good luck finding .300 RUM ammo easily. Admittedly, .300 Win is way easier than .300 Wby, but .300 Wby is still relatively common, world wide.
...I still went for it for my .300 R8 barrel because I have enough .300 Wby ammo to last me a lifetime, but with a decent ballistic scope e.g. Zeiss V4/V6 EET (external elevation turret), Swarovski Z6 BT (ballistic turret), Leica BDC (bullet drop compensation), etc. I would now recommend the .300 Win if only for ammo half the cost and umpteenth time easier to find, including at the local ducca.
Just my $0.02
Well, like you and .300Wby I have .300 RUM ammo to last a lifetime as prior to looking at an R8 with a .300 RUM barrel I was going to build a .300 RUM gun (a LH CRF action is in a cupboard nearby) for the Serengeti. Choice ammunition sent me enough 200 grain Accubond rounds to last a lifetime and I have plenty of practice ammo as well. I agree on Z6i BT scopes. My .300 RUM is going to have a Z6i 3-18x50 BT scope on it. I really like the Z6i 1.8-10x42 BT on the 9.3x74R and it worked out great.
One thing that is interesting during this ammo shortage, there were tons of .300 RUM available, but not that many of the other calibers. I stocked up on practice ammo from those.