Ooh so you need literally 100% chance from one object to never consume ammoTo mathematically expound, with the values you've given:
50% from gun
60% in shroomite (20% of the remaining 50% is +10%)
68% with ammo box on top of that (looks like 20% again there, which gives +8% because 20% of the remaining 40%)
75.% with the potion (another 20%, you've got 32% left, so +6.4%)
Still not horrible with everything, but you can quickly see how little each extra item is worth when stacked on top of everything else.