Why rounded down, central line for burst is in priority, for LSW 4 bullets should hit. Or, if more accurate, 2 bullets will fly into target, 2 in its liine, 3 bullets at right line and 3 at left line, so if something between you and your primary target, it'll get 2 bullets, if not, then your target will get 4 bullets, each with x% to hit, where x is your % shown for 1st bullet, so it's 0.95^4 that all 4 bullets hit target or 81,45%.
For avenger minigun it's rounded up too, so 40/3=->14, % that all 14 bullets hit is 0.95^14=48.77%.
Damage is, as example avenger minigun with AP ammo vs brotherhood armor:
Base damage
((Min;Max)+2*BRDlvl)*DamAdj;
Target's damage reduction:
DR: (ArmorDR-AmmoDRmod)/PiercingMod;
DT: ArmorDT/PiercingMod
Number of bullets:
max/3 rounded up.
Damage: (Base damage*DR-DT)*max/3, if all bullets hit.
or with x2 BRD and no toughness at target:
Base damage: (12;15)*2/3=(8;10)
DR: (40-35)/3=1.66%=0.02
DT 8/3=2.66, don't know where to round.
((8;10)*0.98-2.66)*14=(72.52;99.96)
Damage is 72.52-99.96, if all 14 bullets hit. Also I don't know if DT rounds up/down or rounds at all, I picked it as 2.66
If target will have x2 Toughness, then:
Base damage: (12;15)*2/3=(8;10)
DR: (50-35)/3=5%=0.05
DT 12/3=4.
((8;10)*0.95-4)*14=(50.4;77)
50.4-77 damage, if all 14 bullets hit.
I think everything is right here.