I know a lot of people have posted possible "fixes" for Melta and Plasma weapons. I started thinking "It's a game. Do the mechanics of imaginary weapons necessarily have to conform to known physics?" With that thought in mind, I approached Melta and Plasma weapons from different perspectives, namely what we know of them from "fluff" and the TT game.
Melta weapons are known for punching through armour. One proposed "fix" (as seen in Black Crusade, and I'm assuming will be in Only War) is to double the Penetration value of Melta weapons when fired at a target within Short Range. Their Damage is already considerable, so this seems a logical approach, considering they're known for punching through armour.
As I started looking at Plasma weapons I touched upon two interesting thoughts: "What if it weren't all about how close I am to my target?", and "Melta" and "Plasma" as Weapon Qualities.
In regards to Plasma, I crunched some numbers (the old-fashioned way, mind you) and came up with this…
For each full 3 DoS on the BS Test when firing a Plasma Weapon the wielder will add the weapons Penetration value to the Damage calculation, up to 9 DoS (or 3x Pen).
Imperial Plasma weapons have a Pen of 8-10 (sometimes more, depending on setting). I didn't think it unreasonable, with enough DoS, that a Plasma weapon could do an additional 24-30 points of Damage. A weapon that hurtles miniature suns into a target? Sure, why not? Plasma weapons are rare, sometimes very old or even ancient, so it's not like a prevalence of Plasma weapons suddenly creates a far more lethal game (or maybe it does and I'm just blowing smoke…idk). I suppose if you considered the possibility of Eldar Star Cannons it could be a lethal prospect. But what TT Marine player hasn't blanched at the sight of Star Cannons (at least in the 3rd Ed. Codex…). And Plasma weapons have a specific battlefield role, mainly to combat heavy infantry and light armour (Rhinos, Chimeras, etc.), so upping their Damage, represented by skilled and trained wielders, seems reasonable.
Tau weapons such as Pulse Rifles, which utilize plasma technology and have Penetration values of 4-6, suddenly look more appropriate in terms of their power.
So, back to Melta weapons.
What if the same principle was applied to Melta weapons?
With Penetration values of 4, 8, or 12, I don't think it's unreasonable that a Melta weapon could do an additional 12 Damage (the Infernus Pistol), or an additional 36 Damage (the Multi-Melta) with enough DoS. AND, what if this same principle was applied to the current "fix"? An Infernus Pistol at Short Range would have Pen8, and with enough DoS would do an additional 24 points of Damage. A Multi-Melta would have Pen 24, and do an additional 72 points of Damage. Now we're cookin' with melta!
Other factors that I took into consideration were Range, RoF, and Weapon Qualities. Plasma weapons have the edge with Range and RoF. Melta weapons have the edge with Weapons Quality (Melta, double Pen at Short Range). as a GM, I would consider removing the Blast Quality from the Multi-Melta and adding it to the Plasma Cannon (making the PC Blast), and that should bring everything into focus, as far as what we know from "fluff" and TT about these types of weapons. I would also consider capping ammo supplies for Plasma and Melta weapons. I know the BRB says that Marines carry enough ammo for an entire Mission, but I think this was meant to reference Boltgun and Bolt Pistol ammo, not canisters filled with volatile pressurized fuel.
The Heavy Bolter is great vs. Hordes.
The Plasma Gun is (now) better vs. heavy infantry and light armour.
The Meltagun is (now) better against heavy armour.