Stuff.

MOAR.

STOOF.

MOARER.

STUUPH!!!!!!!!

Few things.
1. What if someone just grinds low tier areas to try and get the weapons? That doesn't seem fair that someone in tier 1 has the same, if not better, odds of getting one of these 4 star drops from the mobs as that of someone in tier.
Solutions: Make it the weapons are only dropped in tir- tier 2 has very low odds of dropping while in tier 3 has slightly higher, but still low odds.
2. Odds of things dropping seem a tad high. Maybe lower to around 1.5 for tier 2 and 5 for tier 3?
3. I felt your idea of the mail gun was both a bit cool and a bit... odd.
Concept art pic 1 Concept art pic 2
So yeah, how about we change the 4 star to a rough, aged badly, looking gun named the "spam mail" that has shadow damage. It could be made into a 5 star shadow gun that is cleaned up, the words fixed to not be such a blur, devilite emblem fixed, colors restored, ect. named the "Silk Express". Or it could be made into a normal (or piercing) damage gun called the "Spiral Orders". The color scheme could be grey mail container (purple thing in pic), white gun part, blue letters (example of letters on left side of first pic) and grey handle with the horse emblem of the spiral order.

I'll draw up concept art for the other weapons and monsters latter as it is late as I am typing this.

4. You didn't design charge attacks for 2 thirds of the weapons so here's my idea for charge attacks for them:
"Spam Mail"/"Fast Sender"
a. Player has a their dash avalible evey 2 seconds for the next 10. If the player switches weapons or gets hit during these 10 seconds than the buff ends. During the buff time they also gain an attack speed bonus every time they dash that lasts for 3 seconds. This can stack to maximum! The timmer on the attack speed bonus is reset after each dash. The player can attack (with the mail gun only obviously) while dashing. The charge time to keep things fair should be at least 5 seconds, if not 7 or 10 and the player should move at gravity bomb charge speed.
This is to imitate the fact devlites move around a lot.
b. The player just does the Antigua charge that fires a holographic pit boss with the last shot instead of a burrddd. (@ THREE RINGS, DON'T PICK THIS ONE, IT IS LAME)

"Grave Robber"
a. The player takes this kind of stance and stabs the sword into the ground. This causes many cracks ,similar to those on the graves, to burst forth, pushing back and damaging enemies. If it is the 5 star version skeletal arms should pop out as well through the bright green cracks.
The "blast radius" if you will, should be as large as the Deconstructor radius .
b. Torika charge attack but spooky spooky green crack *rolls eyes* (@ THREE RINGS, DON'T PICK THIS ONE, IT IS LAME)

Ugh, that's not rare at all. What's the point of rares if they're not rare? I have no idea what the drop rates were before Mastereeve said what he said, but for comparison, the Impostocube spawns in the place of a regular Jelly with a 0.1% chance, and drops the Toupees and Dapper Combos with a <10% chance and there's still lots of those floating around.

Let's say a Wolver's Den has roughly 50 bushes.
With a 5% bush spawnrate that means you've got:
8% chance of finding none (0% chance drop)
So that's a 92% chance of finding at least one:
20.2% chance of finding exactly one (2% chance drop)
26.1% chance of finding exactly two (4.9% chance drop)
22.0% chance of finding exactly three (6% chance drop)
13.6% chance of finding exactly four (4.7% chance drop)
6.6% chance of finding exactly five (2.7% chance drop)

So every Wolver's Den run you do, you have a 20.3% chance of obtaining a Canopy Bomb.
That's a flipping lot of Canopy Bombs on the Auction House by the end of the week after the patch. Rare enough for ya?

The Mailite is a bit more reasonable, but still too common.
Let's say there's roughly 5 mailboxes per Devilish Drudgery.
With a 6% mailbox spawnrate that means you've got:
73% chance of finding none (0% chance drop)
23% chance of finding one (2.3% chance drop)
3% chance of finding two (0.6% chance drop)

So every Devilish Drudgery run you do, you have a 2.9% chance of obtaining a Fast Sender. In a party of four players, that's a 11.1% chance someone is going to pick one up.

And finally the Gravely Robbed.
Let's say there's roughly 20 breakable tombstones per Graveyard.
With a 7% spawnrate that means you've got:
23.4% chance of finding none (0% chance drop)
76.6% chance of finding at least one.
35.3% chance of finding exactly one (3.5% chance drop)
25.2% chance of finding exactly two (4.6% chance drop)
11.4% chance of finding exactly three (3% chance drop)
3.6% chance of finding exactly four (1.2% chance drop)

That totals up to a 12.3% chance of obtaining a Grave Digger per Graveyard run.

But that's why these sorts of "passive rares" are the best. They're not locked to a particular mission or event, so you'll get them eventually through normal play.

Unless it's something creezy rare like the Darkfang Shield, but that's a novelty item anyway.

If the weapons will be functionally different from other weapons, then they should be uncommon rather than rare. The percentages Hexzyle gave are rather reasonable for drop rates (except for that bomb), and especially since they aren't limited to certain timeframes. One bad thing is the fact they start at 4*. At least starting at 3* and dropping in tier 2 would be better, and having a lower chance to get a higher grade item from them in higher tiers.

Special enemies should also have boosted health and perhaps a limited time frame they stay alive so that their drops are a reward. Having them be common would be great if their item drop table was similar to humbug hats dropping from grinchlin stalkers.

When all these are released, there should be a box promo where you unbox one of the weapons at its lowest star rank, and perhaps some confetti and a costume. They would still be free to obtain via the clockworks, but you could take the easier route and buy a box and hope you get the item you want.