"Rarity": {
    "Override": {
      "autoturret": 4,
      "lmg.m249": 4,
      "targeting.computer": 3

Hi,
The higher the number the more rare the item will be

so will the more rare item less possibly spawn?

correct. lmg.m249 is already VeryRare. Rarity 0 does spawn, it's lower then 1(common). there is a list showing default item rarity here: https://umod.org/community/better-loot/44156-what-about-rarity-override-option