Tl;dr at the bottom
The more I play this game, the more I see the balancing flaws. Some weapons are absolutely worthless, while others make the game a cakewalk. For reading's sake, I'm going to compose this post into a list of weapons, how I feel they are now, and what I think should be done to them.
Storm Bolter MK1 + MK2
The storm bolter if a very iconic weapon, it is almost as iconic as the terminator armor itself for Space Hulk, however I don't think anyone will disagree when I say it is one of, if not the weakest weapon in the game. It might as well be shooting paint-balls, especially when you got other guns that can be just as accurate, and deal some damage. I think the storm bolter tries to be a jack of all trades, but fails miserably, as generally a jack of all trades is proficient at everything. I think the Storm Bolter should be a sniper weapon, to capitalize on it's pinpoint accuracy, which really is the only thing it has going for it. To facilitate this, the Storm Bolter should recieve a massive damage modifier on headshots, greater than any other weapon, with everything smaller than a warrior going doing in one shot. Of course, I feel like this could be overpowered, so I think the Broodlords, Scythes, and maybe Warriors should not recieve an as-large modifier on headshot, perhaps only a 5x, while everything else should be 10-15x. This will make the Storm Bolter an exceedingly strong weapon in the hands of someone willing to master it, while also keeping the heat on the player if a Scythe shows up, or if the Genestealers close the distance
The hellfire is the strongest weapon in the game, aside from the plasma cannon, it has just about everything. Damage, AoE, Stunlock on smaller genestealers, and is almost as accurate as the storm bolter. I think this is the weapon that requires the most sweeping changes. What I propose is to keep the Hellfire as a crowd-clearing weapon, just make it so it has to put in work to clear crowds. First and foremost, however, the AoE has to been severely toned down, both for the sake of my eyes, and for balance, the AoE visuals should really only be the size of a Genestealer head, and as for the damage radius, make it so that the AoE only really effects one, maybe two adjacent genestealers. Secondly, decrease it's damage so it's doing only slightly more damage per body shot than that of a regular storm bolter, but to make up for this, give the Hellfire 50% more ammo, and a fire rate comparable to the Storm Bolter. In addition, tone down the accuracy, and make it so that only headshots have a chance to stun targets (Given the fire rate I think it should have, it should only have a 25% chance to stun anything smaller than a Warrior, a 10% chance to stun a warrior, and no chance to stun giants), while not applying the Storm Bolter's headshot bonus, to keep the Storm Bolter from being completely overshadowed by it. I feel like these changes will bring it in-line with non-heavy weapons, and emphasize Apothecary's support role should they decide to use it, the major downside, of course, being that the hellfire will barely be the same weapon.
The plasma cannon is mostly fine - It is a heavy weapon, after all, except I'd change it to only headshots guaranteeing a stun on Scythes and Broodlords, body shots should only carry a 25 - 40% chance of stunning, so that the speed modulator is useful outside of friendly fire servers. Also maybe make it so that any small targets caught in the blast do not get staggered, only direct hits should stagger enemies. Aside from that, make it considerably more inaccurate when moving. I feel these changes will make it less of a cheese weapon, and reward players who get good with the weapon, and not punish the players who are already good with the weapon
Lightning Claws are the only melee-focused weapon that non-assault terminators have access to, so I understand why it pales in comparison to all other melee weapons, however I feel it should still be a useful tool in the face of other melee weapons. I suggest that when letting an attack loose (Not holding or charging one, but mid-swing only) it Autoparries anything that hits the terminator, save for Scythes and Broodlords. I feel like this will make it unique from Purgatory Sword / Thunder Hammer as it still won't one-shot everything, and it will not mitigate all damage like the storm shield does, but it will keep the Lightning claws feeling like lightning claws, with emphasis on being a striking weapon, keeping combat dangerous, and more fast-paced for lightning claws users. That, and you won't have to literally blind yourself to block an attack, being as the parry animation raises your claws so you literally cannot see anything in front of you. Perhaps these changes will even make Lightning claws a viable choice for Assault, although I feel like the storm shield and being able to clear crowds in a few strikes with Thunder Hammers and Purgatory Swords will always be the main draw for assault.
A little powerful, make the flames darker, and perhaps translucent, make it go through ammo faster so you can't just flame out a hallway for five minutes. This will make the flamer a better weapon vs Warriors, Scythes, and Broodlords, but will force the user to have some trigger discipline when facing down larger hordes. Also remove any stuns it deals, I feel like the fire rate increase will mean the Heavy Flamer will still hold it's own down hallways. Aside from that, it's a really good weapon.
Make it so that the accuracy isn't lowered much when moving, making it the sort of run-and-gun go-to weapon. Aside from that, it's a really good weapon, and I don't feel it needs any balance changes
Make it so the AoE does not extend behind the terminator, making it a usable weapon in FF servers, while making it less of a cakewalk for non-FF servers
Mace of Absolution
Keep the AoE as it is, so it will have it's own use, and not be a discount thunder hammer, allowing the Mace Wielder to destroy targets behind them is a major boon, and I feel like the only way to make it viable without a total rework is to change the thunder hammer.
Spear of Caliban
Should be almost the same as the Hellfire, except it deals better AoE, and has better accuracy, at the cost of a funny-working magazine and being unable to stun enemies
Vengeance + Assault Cannon
The Assault cannon and it's variant are really good weapons, and don't need much change. Firstly, make it almost as accurate as a Storm Bolter when standing completely still (Recoil remains unchanged, of course) and give it a small headshot buff compared to what it deals now. In addition, they should become wildly inaccurate when moving and firing. This is to keep the Assault Cannon true to the feel of the original space hulk games, while retaining it's viability, as well as making crusader a more useful skill to activate for tight situations
I feel the changes I've listed will help make weapons more distinct from each-other, while making them more viable, as each weapon having their own role will help prevent balancing issues from arising.