User blog comment:Warl0rd13/Night Lords Tenth compnay First Claw(Warhammer 40k) VS Noble Team(Halo Reach)/@comment-5138827-20121228174443

"Heavy autocannon fire sprayed the roof of the building, churning up its pebbled surface and shredding human flash. The men who had been awaiting rescue in the flyers were the first to die, ripped apart in seconds by the heavy caliber, armour piercing shells. Vedden screamed as an autocannon shell clipped him, instantly shearing his leg from his body in midthigh. He collapsed, dragging the girl to the ground with him.

The Ultramarines scattered, firing at the ornithopters, but their bolter rounds were ineffective against the armoured undersides of the gunships. Learchus sprinted forward, diving to the ground to gather the girl in his arms and rolling on top of her as the ornithopter’s shells ripped towards her. He supported his weight on his elbows so as not to crush the girl and felt the powerful impacts hammer into his backplate. He offered a small prayer of thanks to his armour for standing firm against the traitorous fire. - Nightbringer, page 161"

http://warhammer40k.wikia.com/wiki/Autocannon Autocannons, which are more powerful versions of the M274 Machine Gun,  do nothing to their armor. Good Luck arguing any Spartan short ranged or medium ranged weapons could do it, or the machine gun.

Space Wolf Omnibus, page 11:

He checked his armour to ensure the integrity had not been breached. Here and there were a few scuffs where shrapnel had bounced from the hardened ceramite of the carapace. In two spots he found blisters on the paintwork that told of the fleeting kiss of a lasgun beam.

Full powered lasguns blow off limbs with their intensity. Don't think there should be an argument of sniper rifles doing it either. Furthermore though for humor value I want to point at a fanfiction one of those who researched Spartan and Space Marine armor did:

http://twigknight.deviantart.com/#/d35fgiv

I like the Spartans, but they have no chance here.