Choose one rifle and pistol cartridge.
This cartridge is now the only ammo you can ever fire.
It will serve every purpose from hunting to plinking to defense.
OP here, I forgot to mention that you get a lifetime supply of the ammo.
Availability won't be an issue.
300 Win Mag and 44 Magnum. That should do just fine for me.
13mm gyrojet and 5.45x18mm
>all the fun you could have with unlimited gyrojet ammunition
Keep in mind that most of the problems were due to shit QC, and due to how the ammo functions you could build a safe gun that fires it out of LEGO bricks.
That's my pick.
300 win mag and a scope can reach out and touch anything that's an honest threat.
44mag in a revolver or lever action can eliminate anything that's within pistol range with confidence, with a neat little flare out the muzzle to boot. Sadly I'm fairly loud, slow on my feet, half deaf, and can't see for shit in the dark so if im threatened (animal or otherwise) my only option is full on assault. 44 mag is more than enough bang for hunting too.
7.5x55 GP11 and 10mm. I'd be happy with just that.