I have been researching vulnerability of stormtooper's armor to conventional ballistic firearms for some time and I'd like to hear opinions from people more competent than me on this topic. I am probably wrong in many things so feel most welcom to correct/criticize me.

From what I’ve found out:

1. Stormtrooper armor roughly corresponds to armor class III-A or III and can withstand up to 2000–2600 joules of kinetic action. From what I understand, it’s a polymer composite and is somewhat similar to Kevlar in structure.
2. It is specifically designed to counter blaster bolts and kinetic impacts of shrapnel or fragments, meaning it’s mostly meant to dissipate the energy and heat from the bolts.

Based on this, a 7.62x51mm NATO round (especially the M993) with a kinetic energy of 3500–4000 J is more than capable of penetrating/shattering/damaging standard stormtrooper armor. By this logic, a .50 BMG (~20k J) would overpenetrate it completely, let alone 14.5mm, 20mm, 23mm, and so on.

Given that we’ve seen stormtroopers taken out of action by blunt force trauma (rocks, clubs, traps), we can assume that even if their armor manages to stop bullets of these calibers, the blunt trauma alone would be enough to neutralize the combatant.

Durasteel

The density of durasteel is estimated differently, ranging from 2000 to 8000 kg/m³ by various sources, while MIL-DTL-12560 Class 4 is roughly 7000–7850 kg/m³.

The hardness of durasteel can be anywhere from 400 to 700 on the HBW scale, whereas MIL-DTL-12560 Class 4 is 400–470 HBW. This gives us the following:

The 7.62x51mm NATO M993 penetrates between 12 (at 100m) and 14.5 (point-blank) millimeters of MIL-DTL-12560 Class 4 armor. Accordingly, the same round could theoretically penetrate (I'm completely sure of it) 10mm of durasteel (maybe less, since it’s magical futuristic stuff).

Consequently, a G3A3 or an M14 loaded with M993 rounds can counter a stormtrooper in infantry engagements. As for .50 BMG, 14.5mm, 20mm, and everything else—it's an absolute wrecking.

Vehicles

HEAT equivalents work less effectively against vehicles (from what I understand, it's a similar technology meant to counter incoming energy and heat), just like anything shaped-charge/thermite/incendiary.

Judging by what we see in the movies, even a single Abrams firing depleted uranium rounds would likely put something like an AT-AT in a very awkward position if not dominate it completely, let alone Javelins and other stuff like missiles and heavy artillery. Smaller vehicles wouldn't survive at all, especially utility or cargo vehicles (a speeder or a truck could be disabled with primitive rifles)