So I was with boone. I had equipped a fully repaired Gauss rifle on him. So we're fighting a pretty intense battle and he goes "this weapon doesn't work anymore". I had went over to take a look at it and it was completely broken. He throws it on the ground, picks up a plasma caster, starts using that, throws it, takes out his own rifle and says "this caliber isn't big enough", saw him change to a hunting shotgun, when and where he picked that up I don't know.

So...... His specialty is guns. I don't know why he went nuts and started using all different kinds of weapons maybe just a new thing in fallout NV but one thing I didn't understand is if his specialty is GUNS why is he using Energy weapons over his default sniper rifle ??

I assume it's because the game does a calculation on which of his weapons he will do the most damage and the plasma caster came out on top

Companions will use whatever weapon has the most damage in their aresenal. Most of the time, this is going to be the weapon they came with, as most are unique, already modded weapons.

Now, if the companion can not pentrate the DT of an enemy, like when Veronica complains that the 10MM pistol I gave her is no good against Deathclaws, they will switch out their weapons that they have in inventory. However the Damage of the weapon is what determines what they go with, or should determine it.