Round 1: Hard to say since this version of Snake has been consistently shown to be able to dodge bullets including Sniper Wolf's PSG1 Rifle. Chief has been able to react to high velocity rounds while in armor. Outside of armor he's been able to react to a mini gun but it was firing stun rounds so we have no idea how fast the bullets are.
That being said, unarmored Spartans are more mobile than Snake as they can run faster and jump higher, are better shots than Sniper Wolf and have better eye sight and senses than Snake. Also the UNSC assault rifle is faster than the PSG1, obviously has a faster rate of fire being an automatic and has a range of 1000 feet.
If this fight takes place in Nuketown Snake is going to have a bad day trying to avoid the assault rifle as this is a close quarter map.
I'd give it to Chief
Round 2: This one really sways heavily into Chief's favor simply because of how hard he hits. Chief without armor was able to kick a 2 ton Exoskeleton 8 meters away and has casually dented titanium. Snake could probably dodge but he's going to get hit eventually and when that happens he won't be getting up from it
Chief wins for the above reasons as everything he has gets amplified by 5x. The only thing Snake has going for him now is that chaff grenades may be able to knock Chief's shields offline but it's not going to be enough
Round 2: Same as Scenario 1 but against a faster, stronger, harder hitting Chief.