I think the worst thing I equipped a companion with was a legendary gatling laser and 20 fusion cores as ammo.
A gunner had been using it against my character at Quincy doing a lot of damage to me in a cross fire situation, to the point where I had to retreat temporarily, circle around and ambush them.
I thought the companion would put it to good use but they weren't as accurate as the gunner.
Unfortunately when I took the gun and cores off the companion I had 20 partially used cores as, unbeknownst to me at the time, the game always loads a full one when equipping the weapon.
Partially used cores in my inventory is what drives me crazy so I sold them and gave the gun to a settler.
Also using guns on the hardest difficulties results in greater ammo use and i tend to go on long excursions around favourite hunting grounds.
One favourite is on the Western part near the border of the glowing sea.
First the raiders, then the Mirelurk, then the big Wasteland creatures near the edge of the sea, then the mutant Warlords, then the random encounters, finishing up with the gunners camp in the area before heading back to Graygarden.