I ran the numbers on the classic heavy weapons for my Tacticals/Devastators and the Multi-Melta really is the standout tank buster in 9th:
On the move:
And then standing still:
I've included different unit sizes of basic infantry to allow for Blast, and capped the typical marine statline at 2 wounds. The next couple at 3W are to reflect things like bladeguard and gravis infantry while the three profiles at the end with higher toughness and xW are intended to be multi-wound tanks, monsters etc.
The 'Wounds' values are expected wounds dealt after saving throws and damage is applied (based on average/mean rolls) while points efficiency is simply expected wounds / model points, based on the weapon cost on top of an 18pt marine.
It's also based on the basic weapon with no auras/buffs factored in, so some units with stratagems etc. may find certain options get a boost.
My main conclusion, as has been touched on, is that 9th doesn;t have the same mix of efficiencies that we had in 8th. Before we had heavy weapons ranging from 8 to 28 points with lots of granularity between them, which resulted in some weapons having higher output but being less efficient due to cost. In 9th that's all changed; Many are 10pts, some are 15 and the Multi-Melta at 20 is only slightly higher. This much tighter gap in weapon costs results in those with the higher output still typically being the most points efficient.
The Multi-Melta does shine most within 12" but even at 24" it is still much more efficient than the Plasma Cannon once you're hitting anything with 3+ wounds. The output and points efficiency of the Multi-melta against vehicles/monsters is way ahead given current points costs and it's certainly something I'll be putting on any mobile unit (land speeders, attack bikes, devs popping out of a transport etc.). I can see the points being increased as there's simply no competition in this role. The poor old lascannon pays a premium for the extra range but as a single shot it's far less efficient.
The other standout is the Grav Cannon with the 4 shots making it incredibly versatile. It has the shot count to be decent against basic infantry and the damage modifier to still be efficient against tougher targets. It's the perfect marine killer.
The poor plasma cannon just seems to be outshone by those two, even with Blast. I simply see no reason to take it versus the grav cannon for an all-round weapon considering the latter has no risk of blowing up, hits harder against most targets and is more points efficient, and it's way behind the Multi-Melta that similarly has no risk. To me it feels backwards - I feel like the plasma cannon should have the highest damage potential given it's the only weapon that implodes, making it a real risk vs reward choice. Right now there's too much risk for the low reward.
Oddly the potential role I could see for the plasma cannon is if you expect to face a lot of 1W infantry in squads of 6+ (where Blast would kick in) and you want a squad that can sit still somewhere while firing out to 36", which would make it more efficient than a grav cannon that has to move. Possibly an ideal Sister-slayer? The problems come when you want to overcharge for anything 2W+, as a single hit roll of 1 will completely destroy points efficiency given the cost of the model and weapon you're losing. If you have a Captain to babysit that's then another sunk cost and still no guarantee you won't roll a 1 again.
Fighting a bunch of Sisters in squads of 6-10 is the main niche I see for the Plasma Cannon if it gets to stay still. Once overcharging for the 2D is required I'd rather have the Grav Cannon (versatility) or the Multi-Melta (vastly higher output and efficiency).
Edited by Thoridon, 14 October 2020 - 12:43 PM.