Basically anything above 375H&H is a hand loading proposition really. Even if you're willing to take the financial pain of factory ammo, your supply will be patchy.I was curious to know what the prices are like on these larger calibers so I looked up local prices (Canada) for comparison. These are the lowest prices that I could find.
505 Gibbs - $320/box (10 rounds, $32/round) - Yikes.
500 NE - $210/box (20 rounds, ($10.5/round) - Not bad.
And...that's it. Not seeing any 500J, 577's, 600'sm 700's etc. I'm guessing many of the owners of these cartridges are invested in handloading as retail options appear to be limited.
As a general rule, anything above the .308 range of cartridges is better value when hand loading, but with the big bores, when your standard range time is probably only 10-20 rounds a session, you can make up 50 in a couple hours and doing so saves nearly $500... it starts to become really compelling...
With my .375 in the UK I could buy the rounds for roughly $6 a shot, or I could load them for about $2.75. That sort of saving pays off reloading kit real fast.
Moving up even further, you can probably load a 500NE for around $6 a round, maybe even less for practice rounds. Save 4 bucks on every trigger pull. $50/hr of your time spend loading 'em probably isn't awful.
The Gibbs might be $10 a round and you save 22 bucks a round, or $550/hr of your time spent at the bench. Many would take that.
At the top of the totem pole, anyone who has something really niche like a 700NE who isn't handloading, is either extremely wealthy (and has a personal assistant to scour the internet for ammo), or simply doesn't shoot it much. Probably both, honestly!