The best weapons in Space Marine 2 are the ones that you enjoy using most—in the end the game is just a 40k power fantasy about tearing into aliens with an arsenal of destruction. Why limit yourself? It's not the kind of game where you'd expect a meta to appear for the most part, unless one loadout in-particular ends up dominating PvP, but that would take the fun away, honestly.

If you are a bit curious as to the relative differences between the game's *checks sheet* nine different bolters, as well as its various melee weapons, this PvE tier list should help you out. Some of these weapons are only available in campaign, while others are only accessible for certain classes in the Operations mode.

I also haven't included heavy weapons, because if you see them in the campaign you should grab them—they don't take a primary weapon slot—and if you're playing as the Heavy in Operations, the heavy bolter is the best choice unless you're going to fighting super close quarters, something the Heavy isn't really designed for. 

All that said, here's my tier list for the best weapons in Space Marine 2. If you're simply confused about how to upgrade weapons, then check the section at the bottom of the list.

Space Marine 2 weapon tier list

S-tier

Bolt Rifle

The standard bolt rifle is the best bolter in Space Marine 2 for my money. It's highly accurate, has decent ammo reserves, and is the highest damage variant of the gun except for the stalker. In campaign you can also get the grenade launcher version, which lets you launch grenades into tyranid swarms with its alternate firing mode, and reloads said grenades from regular ammo boxes.

Heavy Bolt Pistol

While the plasma pistol might hit harder at full charge, the heavy bolt pistol is your best option for damage and ammo efficiency. Though it's hard to tell if Gun Strikes are affected by the damage of your weapon, having a straightforward high damage sidearm with plenty of bullets to whip out is still very useful.

Chainsword

Though not quite as speedy as the combat knife, the chainsword makes up for it with extra damage and a moveset that's fantastic for making space in the thick of fighting. If you hold the attack input after a slash, you'll perform a shoulder barge, a kick, or a stomp that knocks back enemies, clearing space, and sometimes making them vulnerable to Gun Strikes. The speed of these attacks make them pretty effective.

A-tier

Combat Knife

The fastest melee option in Space Marine 2 is perfect for duelling against tyranid warriors or special enemies. If you hold down the input you can also slash in wide arcs, which is more effective for groups of smaller foes. I'd especially recommend this on the Vanguard class in Operations since its speed makes it good for an aggressive melee playstyle.

Thunder Hammer

Just like the power sword, the thunder hammer is only available to one class in Operations and it's the perfect complement to the Assault Jump Pack. You launch yourself into the air and raise the hammer, before slamming it down into the ground with a big shockwave. This melee weapon is slower than the combat knife but extremely versatile, with wide swings that will plow through hordes, and charged AoE strikes that deal decent damage. Definitely one of the strongest melee weapons—it's just a shame that only one class can use it in Operations.

Auto Bolt Rifle

Somewhere in between the bolt rifle and the bolt carbine, the auto bolt rifle will suit you if you're looking for a faster fire rate and slightly beefier ammo stocks than the regular bolt rifle. The bolt rifle does slightly more damage, but maybe you'd prefer something faster to unload into swarms.

Bolt Pistol

Though it deals less damage than the heavy version, the standard bolt pistol is a solid option for ammo efficiency and damage—perfect for pulling out when you have your melee weapon drawn and need a quick bit of damage to finish off an out-of-reach ranged enemy or shoot a spore mine or pesky cultist.

Heavy Bolt Rifle

This gun places somewhere between the auto bolt rifle and bolt rifle in terms of how it performs. The heavy bolt rifle has a big mag, decent ammo reserves, and a slightly faster fire rate than the bolt rifle. While it can be used to hit individual enemies, it's more a weapon that you unload full-auto into a swarm. If you want something similar to the regular bolt carbine, but are put off by that gun's lack of accuracy and low damage, I'd recommend trying this.

Instigator Bolt Carbine

Very similar to the marksman bolt carbine except that it fires in a burst rather than single shots. Still, it's very ammo efficient, does excellent damage, and is definitely one of the best and most versatile scoped weapons in the game.

Marksman Bolt Carbine

The best version of the bolt carbine pushes it more in the direction of the bolt rifle, giving it increased damage with a lower fire-rate and better accuracy. This is also one of the best scoped weapons, since unlike the bolt sniper rifle or las fusil, it actually has decent ammo reserves to fall back on. Sadly, it doesn't seem to be available in Operations, though you can grab it during campaign co-op.

B-tier

Stalker Bolt Rifle

Despite its damage, the stalker bolt rifle has the same problem as other scoped weapons like the las fusil and bolt sniper rifle, which is to say, small ammo reserves. While the marksman bolt carbine and instigator bolt carbine are both scoped weapons that feel fine for general use because their frame has higher ammo reserves and a faster fire rate, you'll burn through the stalker's ammo reserves very fast.

Melta Rifle

Though it has very low ammo, I really like the melta rifle when used with a melee-focused class like the Vanguard. You can alternate your fast combat knife strikes with close-ranged blasts from this, treating a bit like you would a shotgun. That said, if you aren't going to be using a grappling hook to get up into close range with the enemy fast, you'll likely want a gun with a bit more range.

Plasma Pistol

The plasma pistol does very good damage when you charge its shots fully, but the problem is that it has horrible ammo efficiency. If you fully charge each shot, you'll only get about ten from full ammo. Sure, you can use it not charged and it's decent against armour, but I'm not sure that's enough to put it above the heavy bolt pistol, or the regular bolt pistol.

Power Sword

The power sword only seems to be available to one class in Operations; the shield-wielding Bulwark. Though not quite as fast as the combat knife, it packs more of a punch, with two different attack modes you can use. Similar to the combat knife, you can also hold the melee input to use powerful arcing slashes that are better for dealing with groups.

Power Fist

Though I personally love the power fist for its incredibly gory executions and superman punch as you run into melee, it doesn't feel as strong as the thunder hammer or power sword, in-part because it struggles to deal with big swarms. While the thunder hammer has its wide swings and downwards AoE strikes for this task, the power fist has to charged up to hit a lot of enemies, and while you're slowly charging you're also taking damage. Still, it's a lot of fun, and definitely viable if you want a weapon with a higher skill ceiling.

Las Fusil

The las fusil has less ammo than the bolt sniper rifle, but once you unlock the Lethal Efficiency perk for the Sniper class, it becomes the go-to. This perk refunds ammo for the fusil every time you kill more than one enemy with a shot, partially solving the Sniper's issue of having a small ammo reserves. It certainly feels trickier to use with the charge time before every shot, but if you keep clear of combat and pick your targets, it's a powerful gun.

C-tier

Bolt Carbine 

For my money, the bolt carbine is the worst of the bolters, with a high fire-rate that makes it tough to maintain accuracy, and low damage which means you have to absolutely riddle enemies with shots to actually take them down. It pays to have a weapon with some accuracy in Space Marine 2, especially when you often need to take out ranged enemies at a distance or get peppered with projectiles.

Oculus Bolt Carbine

This variant of the regular bolt carbine isn't all that different from the base version. It deals more damage and has a slightly slower firing rate, but it still lacks accuracy in the same way and can struggle to hit enemies at range. I'd definitely recommend taking the instigator over this where possible.

Plasma Incinerator

The plasma incinerator has the same problem as the plasma pistol. It's strong if you charge it up and seems to pierce armour, but firing a fully charged shot means you only get around 13 from full reserves. Once it's exhausted, you'll be without a primary weapon, which might put you in a tricky spot till you can find some ammo.

Bolt Sniper Rifle

The problem with the bolt sniper rifle in PvE is that it isn't that ammo efficient, especially since it doesn't get the Lethal Efficiency perk for ammo regen like the las fusil.  Sniper rifles often don't feel all that good in a such a melee-centric game, especially when executions are vital for regaining armour and you can't really do that with a ranged weapon beyond Gun Strikes. This is definitely a viable option for the sniper once you unlock more perks, but the las fusil is generally better.

How to upgrade weapons in Space Marine 2

Space Marine 2's weapon upgrade system for Operations is a little complicated so here are the various steps involved. If you head to the armoury you can see the weapons available to your class and pick between them. 

If you want to upgrade a weapon there are two things you need to do:

• Gain Mastery with a weapon by using it. This is pretty straightforward—kill enemies and complete Operations with your preferred weapon until you fill the first section of the XP bar shown on that weapon’s track. This will also grant you mastery points you can spend to unlock perks in each weapon’s perk tree.
• Acquire armoury data by finding servo-skulls during Operations missions and extracting with them. You can also get three master-crafted armoury data by getting A rank in the Trials activity that is available on each class screen,
• Spend armoury data to unlock a new tier of available weapons. These will have different stats and visual appearances to the base version. To unlock the first tier you’ll need master-crafted armoury data, then artificer armoury data, and then relic armoury data, with those last two coming from Substantial and Ruthless difficulty Operation missions respectively. Unlocking higher weapon tiers will also open up the corresponding sections of their perk tree.

That's about it, really. Use the weapon to get mastery points, acquire armoury data, and use both to acquire higher rarity versions of the weapon as well as perks.