No nerf needed for the AR, defend yourself with the SK, problem solved...
Now will you say that the SK is overpowered or that you cannot use it?
The Sidekick sucks. It's optimal ttk is technically faster if you spam it at closer range. But it's a 7sk which on top of being too many shots is over half the weapons magazine size. And even if you have perfect aim while slamming the trigger it has random spread via bloom.
The sidekick is designed not as a reliable sidearm, but as a throw away secondary used almost exclusively as alternative to reloading your primary.
The Halo Reach Magnum was very similar, but at least it was a 5sk instead of 7. And the AR in Reach was much less viable at range, had a shorter magazine (only one kill per mag rather than two), and a much slower kill time.
This is not a balanced loadout. Buffing the Sidekick significantly is a consideration, but the AR still needs a nerf.