Just thought I'd throw out some spec right from the wiki. I hope you have a pen and paper.
M6C: (The Halo 2 Pistol)
12.7x40mm M228 Semi-Armor-Piercing High-Penetration .50 Magnum Caliber Rounds. 12 rounds in one clip.
rate of fire: About 360 rounds/min 6 rounds/sec.
Maximum range: Between 50 and 100 meters.
BR55HB SR: (Battle Rifle)
9.5x40mm KURZ .374 Caliber rounds. A whopping 36 rounds in one clip.
Rate of Fire: Average, 2.4 Bursts per Second, 900 rounds per minute.
Maximum range: 950 meters.
SRS99C-S2 AMB (Halo 2,3,Reach Sniper Rifle)
14.5 x 114mm AP-FS-DS Armor-Piercing, Fin-Stabilized, Discarding Sabot. 4 rounds in one clip.
Rate of Fire: Average, 2 Rounds per Second. Semi-Automatic.
Maximum range: 2300+ Meters.
Now if you want to add all these extra mods to the modern guns then stop right there, Halo is 500 years in the future who knows what kinda of weapon mods they have. I am just making a bone stock comparison of the modern weapons you listed and the weapons found in Halo.
Now time to compare:
First off want to start by pointing out that you shouldn't compare the AR to the M16 A4. It would be more suitable to put it side by side with the BR.
M16 vs BR
Maximum effective range:
M16: 800 meters < BR: 950 meters
Rate of Fire:
M16: 800 RPM < BR: 900 RPM
Caliber size:
M16: 5.56×45mm < BR: 9.5x40mm
The victor here is quite clear. The BR wins by a landslide thanks to its increased Caliber size and range.
If you can find any weapon that exists to date and that can beat these 3 Halo weapons above let me know. But right now I feel like I have put in too much time trying to find all this info and I would currently like to focus my time else where, tho I will come back to this topic, just not right now.
And you may want to keep in mind that this is a video game and that the developers have to balance the weapons according to gameplay which is why the shotgun in all the Halos are pump-action. If we had the best of the best weapons that exist to today in Call of Duty it may not be that fun/fair.