well unless you have a s@#ton of other than NATO or soviet standard ammo shtf means you will end up using that system pick.the best you can 7.62×51 mm lots of it. 5.56 is good for lmg against infantry it keeps heads down. Sure a 50 would be nice to have but nothe mobile at all without support and vehicles for long distance.
3006 can’t shoot that for days and still use your shoulder those older m1 steel plate shoulder kicked like a mule 500 rounds in a day would be a chore. Let a alone for days and moving with that much on foot would be limited to one maybe 2 engagements before you are out of ammo.
Also use the hp soft nose 5.56 it’s a different animal that a fmj in same calibre.