Why does PVE bots shoots limbs with tier 1 bullet and tear them down like nothing? And any tips going to northridge?

Ive played a fair amount of PVE solo now and I observed that snipers have golden bullet so it's fair but how the frick does a no color bullet 1 shot a limb and it's so damaged? I get why people get mad and some enjoy it, it's really a great game mode but you gotta admit it's annoying to always heal? But I always wondered how that works