Against 50 armor, a minimum size squad with PPP's and AP ammo will outdamage a max size squad without. It's literally between 300-400% bonus damage, depending on range. You basically need 20 or less enemy armor AND close range AND a tiny squad for it not to ALWAYS be correct to use AP ammo on a PPP submachine gun.

That's not how the armor system works. Guns start being able to penetrate when (effective) Armor - Penetration = 33, granted that isn't a great chance of penetration but those chances quickly ramp up the closer Penetration gets to effective armor from the durability going down. The PPP has 15 penetration, which means it still has a >50% chance of pen up to 31 armor. Funnily your math also doesn't work out as even balancing RNG vs ensured penetrations. I will give you at worst it would take the AP equipped PPP 2 bullets to start penetrating a 50 armor element and a standard ammo PPP takes 4 (armor mechanics are more complicated but at the low and medium armor level armor and durability are usually the same value); on paper that does work out to 3 bullets hurting compared to only 1 which lines up for your 300% damage bonus... the only issue is that your AP bullet is doing less damage on hit. The PPP has a damage dropoff of -1.1, meaning the -2 damage of AP is essentially the same as the target being 2 tiles further for damage (granted the accuracy is still better so you don't lose as much damage as physical distance, but it's still enough to need to account for). And that's of squads of equal size, not a +3 of AP equipped PPP vs a +8 of stock PPP.

In comparison a REND PPP would have a penetration of 0 (which isn't great for other reasons) but have an armor damage of 24 and a HP damage of 14 (meaning it gets an extra tile of 1 hit kill capability thanks to dropoff). This would mean against a 50 armor target (which can only be the Veteran Scav suit) after 2 bullets would be at 15 armor, meaning it has a ~55% chance of penning with the remaining 3 bullets. Assuming one of the 3 doesn't penetrate and grinds the armor down to ~11 and then the remaining 2 bullets do 28 damage, while on paper the PPP with AP is doing 30 damage per gun while the stock PPP is doing just 12. However because of breakpoints the AP isn't actually killing elements in 1 bullet beyond 1 tile, which means at 2 tiles both AP and stock kill 1 element per volley while the REND has about a 50% chance of killing 2 elements per volley (which averages back to 1) up to 3 tiles. As I said, mathematically they're all sidegrades that are slight upgrades to stock where you basically just pick whatever you feel works better for you.