MK17 is long range but it sucks against body armor because it can't pierce armor reliably (it has the same Armor Piercing, or AP, value as the regular rifles and it fires too slowly)

It's great for one-shotting unarmored enemies but even enemies with pistol-grade body armor don't always get one-shotted, despite having insufficient armor

You might have to flashbang the guys and then hope the armor RNG lets you actually kill them while they're stunned (body armor rolls a dice on whether it blocks the damage entirely or reduces it)

Long range firefights really suck in DK1, there's no way around it. I think the real issue is that enemies are just too accurate at long-range. That, and the MK17 should have an AP value of 6 rather than 5 so it could at least do reliable damage against enemies with rifle-grade armor.

I think the P90 and MP7 might also be decent-ish, but you still need to flashbang armored enemies and then hope you can kill them in time before the stun wears off. Rate of fire kinda matters more than damage when you're up against armor, and the SMGs generally shoot faster (at least if you get close enough or are behind cover)