Since in the description, it dosnt say to make a ranged spell attack does that mean magic missile always hits?

Yes, one of Magic missiles Features is the fact that it doesn't need a roll to hit. This makes up for the fact that it does less damage than some of the more popular choices, such as Firebolt which is a cantrip

Yes, Magic Missile always hits. The only way to avoid it is by casting Shield as a reaction.

Shooting at one target only:

Lv1 - 3 darts:
3d4+3 => min: 6pts | max: 15pts | avg: 10pts

Lv2 - 4 darts:
4d4+4 => min: 8pts | max: 20pts | avg: 14pts

Lv3 - 5 darts:
5d4+5 => min: 10pts | max: 25pts | avg: 17pts

Lv4 - 6 darts:
6d4+6 => min: 12pts | max: 30pts | avg: 21pts

Lv5 - 7 darts:
7d4+7 => min: 14pts | max: 35pts | avg: 24pts

Lv6 - 8 darts:
8d4+8 => min: 16pts | max: 40pts | avg: 28pts

Lv7 - 9 darts:
9d4+9 => min: 18pts | max: 45pts | avg: 31pts

Lv8 - 10 darts:
10d4+10 => min: 20pts | max: 50pts | avg: 35pts

Lv9 - 11 darts:
11d4+11 => min: 22pts | max: 55pts | avg: 37pts

Discussing this with my partner - is damage calculated rolling a d4 three times, or rolling a d4 once and multiplying by 3?

d4 three times. You can choose to target separate creatures if you want

Hit or miss. I guess they never miss, huh?

The bigger question is that does each dart count as a separate hit? Does magic missile automatically make a downed PC fail all 3 death saving throws, as each dart is a separate source of damage?

I think so but i might be wrong.

if i am wrong though that means i need to give my DM a little payback.

good thing i save my murderhobo PC's for last >:3

As the spell specifies that all darts strike simultaneously, I believe multiple darts hitting a single target only counts as one hit and one source of damage. So, a downed target only fails one death saving throw and only one constitution check for any spell casting who are concentrating.

According to Jeremy Crawford (the guy that wrote the rules), you roll the d4 one time and use it for all bolts. (Link)

And according to the same guy, each Magic Missile counts as a separate source of damage. (Link)  But as a DM, that's a real crap thing to do.  I would roll a single Death Save (or Concentration) using the combined total counted as a single source.

My Dwarven Barbarian acquired a scroll for this spell and I took the spell sniper feat. I now have a barbarian with Magic Missile and  Eldritch Blast.

Great spell

Does it always hit if the target is wearing a Cloak of Displacement?

Yeah, there isn't an attack roll for the spell, the only thing that blocks it is the Shield Spell or the Brooch of Shielding.

Yes.

Guess what? *Uses Shield Spell*

Add that on top of some broken 21 AC armor, with a homebrew ability that lets me add my charisma modifier to my AC for one minute. 24 AC. Guess what? 29 AC. Almost a "Nat 30"

Wand of MM+ Arcane Firearm feature from the Artillerist= Smart Ammo Submachine gun.

Question is does the Arcane firearm only add ONE D8 of dmg to the total or to each dart?...

When you cast an artificer spell through the firearm, roll a d8, and you gain a bonus to one of the spell’s damage rolls equal to the number rolled.

Hope that answers your question.

Yeah saw it afterwards, bummer.

Well a Wand of scorching rays would still work.