If Master Chief was in the 40k universe one bolter shell would most likely put him down.
Consider one shot from a sniper rifle in Halo punches through his shields and reduces his health to almost 0. Those guns do not fire .70 caliber mass reactive explosive mini rockets. He'd be screwed 6 ways to Sunday. His armor would be a 4+ at the most, his shield would give him a 5++ (maybe).
Secondly, his wargear would be all wrong as we know his battle rifle does not use ammo anywhere near the caliber of a Bolter round so it can't be Str/AP 4. I would say Str 3 AP -
Third: Str 10? Really? You think Master Chief is stronger then a Carnifex? He's str 4 at the very most.
Fourth: 6 Wounds? Does he even have enough body mass to cover that? Creatures like the Trygon, Trygon Prime, Tervigon, and a Trynnofex have 6 wounds, not some lowly mortal. I'd give him 3 wounds tops, should really only have 2.
Master Chief would get beaten down so badly by a Space Marine/nid, /Deamon/Necron it wouldn't even matter.
Then again, fan-boys will jump on me for saying this with the normal "ZOMG SPARTANS ARE THE BEST!112!!!!221! Master Chief could solo the ENTIRE 40k universe."
I'm a massive Halo fan.... but this was exactly what i was thinking whilst reading this thread.
The thing to remember is, It's all relevent. In the halo universe he is very strong (both mentally and physically), he has good equipment (Mjolnr armour and shields), great training (trained from the age of 6?) and access to all weapons at the time. He own's people in his timeline......but 40k isn't his timeline.
The issue is that most people see how well some weapons do against power armour or read about how some armies are great in combat, but seem to forget that a lasrifle is used becaus it is better than a standard projectile weapon or that space marines are 8ft tall and are trained to kill for decades, maybe even centuries in the cases of high ranking SM.
The weapons and tech of the 41st millenium may seem ineffective at times, but thats only because everthing has advanced whilst staying equal. e.g. peope used to use bows and swords to fight wars whilst wearing armour, now we use guns and bombs to fight with whilst wearing body armour. effectively the same, but one example is 100's of year ahead the other.
Also, FYI Halo:reach cotains maily spartan III's where as the chief was a spartan II. Details make all the diffference