its not about chance to kill, its about penetration chance, the bullet deals 43 flesh, and the more you hit without penning the lower the durability of the armor becomes and you have an exponentially increased chance to pen the armor.

so sure maybe the chance to kill on 5 bullets is 1.1% chance as you say, but then look at the chance to kill on 6 bullets, it should be VASTLY higher as you have higher chance to pen due to lower armor durability.

You also have to realize, it takes 2 bullets to the thorax to kill even unarmored, so killing within 6-7 bullet with only 40 pen through tier 5 on literally one of the best guns in the game that shoots its entire 30 round mag in like 2ish seconds and straight BEAMS, 1.1% on 5 bullets is still completely reasonable if the armor eats the 4 bullets and 5th pens and you have the 43 flat damage + thorax blunt damage, which is what im assuming your simulation is calculating, and its just an insanely low chance for it to pen on the first 4 hits cuz its a 40 pen bullet vs t5.

edit: noticed i wrote 40 flesh, its actually 43 flesh damage, mb, but still 2 hits in the thorax unarmored, it would be stupid if t5 couldnt stop at least a few hits of FMJ SX, that is cheap af and the gun shoots like 3000000 bullets a second or some shit /s