Here they are:
Item rarity is tied to machine class with C-class variants dropping the best items
Source: June Update Now Live | Generation Zero
New Gear: Experimental Weapons
To match the challenge involved in taking out Rivals, we’ve created a new tier of weapon for each weapon class that have a chance to drop when you defeat them. As the Rivals get stronger, the better chance you have to get your hands on these powerful weapons.
Source: October Update Patch Notes | Generation Zero
Thankfully, the new Machine Gun class of weapons we’ve introduced can also be found by all players. There is a twist though, you’ll have to defeat some of the new machines to have a chance to get them.
Source: Alpine Unrest Update Notes | Generation Zero
2.Experimental Weapons drop chance should now be correctly tied to Rival level and Machine type. The lowest chance is now a Level 1 Prototype Class Hunter and the highest is a Level 4 Apocalypse Class Tank.
Source: January Update - Now Live! | Generation Zero
And the same goes for what loot the Reaper drops - we’ve shared some teasers and gameplay of the Experimental Sledgehammer that you can get, but that’s not all that can be looted.
Source: November Update - Patch Notes | Generation Zero
With all that, we can learn:
1* 2* proto
4* 5* FNIX
lowest chance with lvl1 proto hunter
highest chance with lvl4 apo tank
Apo class, where:
1* 2* runner
4* 5* tank
And the rest comes from gameplay:
same as vanilla weapos
US and Soviet DLC:
N9, AT-WAD - hunter
N16, SVD - harv
N60, RPG - tank
1* 2* proto
4* 5* FNIX and apo