It would appear it would fall under automatic pistols/rifles depending on stock choice...

I have seen on other sites that it is affected by the heavy guns perk?

if you mean Sentinel's Plasmacaster on wikia, i corrected it already. it is just a unique plasma rifle

As a note: this post isn't assuming you don't know any of this, it's just to answer any questions you or others might have regarding damage perks and weapon classes

Anything with a stock, even a really short stock, is a rifle (this includes every shotgun.) Anything without a stock is a pistol - exception being the hunting rifle, apparently, which I find a shame but I digress.

After determining if it's a rifle or pistol, then the only thing left to consider is whether or not it is automatic, as automatic status takes precedence over rifle/pistol status as far as damage perks are concerned. Commando will improve the damage of automatic shotguns, automatic pistols, and automatic rifles, where gunslinger and rifleman will not as they only improve the damage of their respective semi-automatic classes.

Heavy weapons all have the underslung grip (minigun, flamer, junk jet, etc.)

Rocket/missile launchers fall under explosives.

I have yet to find an instance besides the hunting rifle where these rules are betrayed.

And heavy weapons, its one of the few weapons that can double team, perk wise.

This is something I've been wanting to know since I have a Legendary Penetrating Missile Launcher that ignores 30% of Armour and I've just put a Targeting System on it. It's *cough* kinda decent.

If it's a modified flamer then it's a heavy weapon

Its a modified plasma weapon

http://fallout4.wiki.fextralife.com/Weapons

Is under heavy weapons on this. Probably wrong

Yep it is wrong. I got a big gun build and was looking for it then I found it is just plasma rifle