From StarCraft II Wings of Liberty “The Deal”
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CMC combat power armour. This suit provides the user with armour plating offering immunity to most small arms fire covering most of the body; however hypersonic projectiles fired by a powerful gauss rifle can still punch through.

Liberty’s Crusade wrote:
Raynor was in his own suit, looking equally battered and worn. The lawman nodded behind his open visor and said, “The armor will stop most common slugthrowers, though a good needle-gun can still punch through. That’s why most front-line troops carry C-14 Impalers, gauss rifles that fire eight-millimeter spikes”.

According to the novel “I, Mensk”, only the most concentrated of bursts from the older C-14 gauss rifle MK I could penetrate CMC power armour despite the weapons prestigious rate of fire (45 to 150 shots per second).

The newer and larger MKII C-14 gauss rifle was significantly more effective, easily capable of over-penetrating and coring CMC power armour (as seen in StarCraft II: Heir Apparent).

I, Mengsk wrote
Valerian pulled the trigger, working his fire over the second marine. The recoil of the gauss rifle [MKI] was fearsome, designed to be absorbed by a powered combat suit, which Valerian conspicuously wasn’t wearing. The roar of the weapon was deafening, but Valerian kept the rifle on target, knowing that his target’s armor would defeat all but the most concentrated clusters of impacts.

I, Mengsk wrote
He pushed himself to his knees, and felt a series of ringing hammer blows on his side. He fell back, seeing a pair of green-armoured soldiers advancing toward him. They were good, disciplined soldiers and walked their spikes into him, keeping him pinned with the weight of fire. More red icons flashed up on his visor, warning of imminent armour penetration.