My personal vote is to Umbra Driver but I'm extremely biased towards it. Like Holy-Nightmare said, most Shadow-weak enemies aren't that fast, so similarly to abusing an Autogun's style of attack, you can freely abuse an Alchemer's style of attack, but Alchemers have those extra neutral mobility options that Autoguns lack if you happen to need them. At this point it's whether you want more offense or more safety, and I prefer the latter. Not to say that Grim Repeater is not safe, but Umbra just has those extra options.

On the note of Phantamos, it's really hard to compare to Umbra or Grim Repeater; its defensive usefulness is marred by Umbra, and its offensive usefulness is easily surpassed by both Umbra and Grim Repeater. But it looks hella cool and good looks is always something to think about, you know?

Along with alchemers and autoguns, magnuses have really strong charge attacks. They are really amazing. So, yes, Winter Grave is good. That said, there are two drawbacks:

A. The charge attacks knock enemies back in a weird way. Ideally you shoot the enemy in its front. Positioning yourself to do this slows you down a bit. All magnuses (including Iron Slug and Callahan) have this defect.

B. Winter Grave's freeze frequently stops the knockback of the charge attack and hence prevents full damage. (Skepticraven and Dibsville are going to yell at me for bringing this up.) The effect is lessened somewhat if you're shooting at distant targets. And it still does a pretty good amount of damage. And of course freeze lets you set up another shot easily. But I find crowds of monsters much easier to one-shot-kill with Iron Slug or Callahan.

You should try every style of weapon in Spiral Knights, just for the fun of it. So make sure you have an alchemer, an autogun, and a magnus. And you should have weapons of various damage types, to help you kill enemies in all situations. Plan your Umbra, Grim, or Winter accordingly.

So you want a Blaster/Magnus because you already have an Autogun and an Alchemer, or did you mean something else by that? You can get the same weapon type in multiple elements and nothing will be holding you back... Acheron is not less useful just because you have Combuster, the same as Umbra or Grim Repeater are not less useful just because you already have VP and (are making) Storm. Umbra and Grim Repeater are fantastic choices as a standard Shadow gun. Winter Grave is a great choice if you plan to use it as a type of primary weapon, although you might find more use out of Umbra or Grim if you were using them as a secondary weapon. Phantamos is just kind of there to look cool. Permafroster is niche, and Sentenza/Carbine are just a preference. Yes, experiencing different weapons is very fun and a good way to see what you truly want from a weapon, but you should look at the weapon for its purpose on its own rather than comparing it to other weapons. In essence, compare Grim Repeater, Umbra Driver, and Winter Grave to each other, but don't compare them to Volcanic Pepperbox, Storm Driver, or Callahan.

If you're going purely PvE and not PvP, then Fearless Rigadoon and Furious Flamberge are already out. If someone can legitimately make an argument saying that Furious Flamberge has any use whatsoever besides when you want to do reduced damage for a certain purpose (setting up certain parts of a speedrun), or trying to do a slime/construct area with just piercing weapons, please go ahead. Furious Flamberge is absolute garbage, just going to put that out there.

Fearless Rigadoon rarely procs unless you use the charge attack, but the charge is very unsafe and not the selling point of the weapon. You can get it if you want something interesting but it's really not comparable in usefulness to Barbarous Thorn Blade or Final Flourish.

So really, your choices are between Final Flourish and Barbarous Thorn Blade. Personally, I like Barbarous Thorn Blade better because its charge is an forward-AoE attack that can do quite a bit of single-target damage, as well as having nice crowd control (although it has quite a bit of charge up, I could compare it to something like doing a Divine Avenger charge). The AoE makes it useful against enemies such as Trojans or Maulos, or enemies in large groups.

On the other hand, Final Flourish's charge, although pretty unsafe in most cases, is devastatingly strong, but only if all three hits connect. If only two hits connect, the damage is comparable to BTB's charge, and if only one connects (rarely unless you hit the first attack and go past the enemy so you miss the second two), it's much weaker than BTB. It's quite inconsistent and it's not something you're going to fall back on often.

The catch here is that honestly, both BTB's and FF's charge attacks are very rarely used and both are quite inconsistent in their own ways, so in choosing between them, simply ask yourself whether or not during those rare occasions where you might want to do a charge, would you rather have raw damage or an AoE? As a side note, BTB's charge is great at defensive play and making space (such as if you get swarmed or cornered), while FF's charge is great at offensive play and wall-breaking to make space to move around.

And if you honestly just think that you'll never really use the charges (valid option), go for whichever looks nicer. c;

Also, although less important, Flourish lines are significantly easier to find good UVs for than the Snarble Barb line is. If you don't care for UVs regardless then this point has no value.

"B. Winter Grave's freeze frequently stops the knockback of the charge attack and hence prevents full damage. (Skepticraven and Dibsville are going to yell at me for bringing this up.) "

Yup. Callahan/Iron Slug are better for close range, where they can continually be pushed back and damaged by the charge. WG's freeze makes it pretty awesome for sniping (as there is one additional hit when the projectile itself dies). I find that if you're skilled enough with either of them, the quantity of damage will 1-shot most monsters with both.

While you can get 5-6 hits from a slug/callahan, it is still fairly common to only get 4. WG I can only manage a max of 5 hits. Monsters usually die somewhere between 2-4 hits depending on the party size/difficulty.

I'll also note that on monsters that the projectile doesn't pierce, a point blank attack deals 3 hits. This makes Slug/Callahan fairly good for taking out trojans.

@Bopp

Almost.

"To clarify, this applies to all three magnus versions, right? I haven't noticed any difference among them."

Correct.

"* The monsters at the back of the crowd will feel only the end of the projectile. In your view, WG is roughly as good as IS/Callahan for these monsters?"

WG is better in this case, unless the monster is against a wall. This is because at the end of range, Slug/Callahan will push the monster out of range for the "end of projectile" hit. WG will likely freeze and deal the extra damage from the "end of projectile" damage.

"* The monsters at the front of the crowd will feel the entire trajectory of the projectile (with proper knockback). They are your "close range" case, so IS/Callahan is better for these?"

Correct.

Specifically with slimes, WG is my preferred pick. Gremlins it is about 50-50 since freezing the shielded thwackers is useful, but knockers get up close too fast. Constructs it depends on which ones are present. Retrodes I prefer WG while scuttlebots are a clear IS. Wolvers IS has the advantage, but WG is still very useful. If you're only getting one, go for the IS. If you're making 2, id suggest WG and Callahan.

It was a thread about magnuses for much of posts 4-9. You can't expect us to drop the topic, just because you changed the title.

When you make a thread, you control the content of the first post. But the thread belongs to the community, and the community may take it in a direction that you didn't intend. This is also happens in conversations in real life. You should not be surprised or upset.

Anyway, have you read the advice that you've received about flourishes? Do you have any response?