You can still download Chairgun frome Hawke's website. It is pretty handy to look at different what if scenarios
https://www.hawkeoptics.com/chairgun-and-x-act-end-of-life.html. Not many regulated big bore rifles on the market. I think the challenge there is getting a plenum size larger enough to drive the heavy projectiles. Will have to look at a boutique brand like Amaerican Air Arms.
The problem with supersonic is having enough power to drive the projectile well above mach1 so that there is flight time at supersonic to get to target, Otherwise it drops right back down through transonic and becomes destabilized.
.257 Weastherby Magnum is nice flat shooting supersonic round....lol