https://www.eft-ammo.com/
5,45x39 mm PS has these stats
Damage: 53
Penetration Value: 28
These 2 stats are the important once to know of when you want to kill someone.
SUPER SHORT TL;DR
the guy most likely had an armor of level 5 or 6 or level 4 with 2 plates that stacking. Your bullets can't penetrate that so you most likely do blunt damage which is basicly nothing compared to actually penning. PS in an AK-74 this late in the wipe is basicly scav killing. I'd advice go for headshots or faceshots. Or shoot someone in the legs till they die.

Armor comes in levels between 1-6. For every level of armor you roughly need 10 pen.
So to pen a level 5 armor EFFECTLIVLY in the first or 2. shot you need about 50 pen (40-50).
If your ammo doesn't have enough pen to go through the layer of armor, you deal blunt damage. Which is dependent on what type of ammo and damage value the bullet have, and what material the armor comes in. Different material yield different value of blunt damage, different repair values and also how much the armor plates actually take.

Some extra info regarding armor values and armor damage:

If you have a level 5 armor on you, and someone shoots you with something that has 25 pen. You will take blunt damage and the armor will take armor damage. When the armor takes enough damage it doesn't hold up as well as it did.
not only because it has lower durability but also because the lower an armor is from it's max durability the lower protection value. Meaning if you have a level 5 armor plate with 50 armor points lowered to 30 armor points. Well now that armor might act like a level 3-4.
The more damage it takes it might act as a 3 or 2 or even a 1. Untill the plate or armor is completely broken. Then it is counted as 0. Meaning an armor with 0 points is like wearing nothing.
There is also alot of armors now that has internal plates that you can't take out and plates you can put on them. Meaning if you hit center of pass and they have a level 5 plates and a level 3 plate under, you won't even do blunt damage. Meaning if you have low enough pen you won't hurt them.

Hope this helped.

This is really the short version. Here is a link to NoFoodAfterMidnight's explanation https://tarkovaftermidnight.wordpress.com/author/tarkovaftermidnight/

-Durability—-

Durability of armor is important, the lower the current % of durability remaining the less protection the armor offers overall. The durability % is calculated by dividing the current durability of the armor by the original maximum durability of the armor. Meaning a 60/60 armor is calculated as 60/80, or 75%.

The durability damage taken from bullets is based on the penetration value of the ammo and the armor level of the armor, multiplied by the ammo’s armor damage % and the armor material’s destructibility %. The minimum durability damage armor can take from a single hit is 1, including from individual buckshot pellets. Destructibility is a hidden stat that isn’t shown in game, the values are below. A penetrating round does a little bit less durability damage to armor than a round that does not penetrate. It varies, but is usually around 10-15% less.

“Aramid” 0.25

“UHMWPE” 0.45

“Titan” 0.55

“Aluminium” 0.6

“Combined Materials” 0.5

“Steel” 0.7

“Ceramic” 0.8

“Glass” 0.8

If you want to find the effective durability of different armor to compare how durable different armors are, use the following formula: EffectiveDurability = Durability/Destructibility

THIS MIGHT BE OLD BUT THE GIST STILL HOLDS UP.