I have been playing generation zero since it came out and play every day at the moment with my son. I love the game and am getting to grips with the new changes . However I know they changed the rival system in the previous update but was excited to hear there are new items added to the rivals in the March update. However we have killed three apocalypse tank rivals and all I’ve picked up is some ammo , rockets etc and if I’m lucky a green combat jacket. Does anyone know what the ratio of actually getting an experimental weapon is ? I’m feeling disheartened after killing loads of robots to create a rival and then to be given a ruby coloured bike. Also one last moan , I’ve been killing rivals for months in the search for a red experimental dazzle jacket but no. And then my son gets two of them and not one experimental weapon I have been blessed with. Sorry for the whinge but I’m all out of fight.
It’s totally random, but higher the level of a rival - higher probability of getting something in return.
Does difficulty matter on the drop rate for event?
No worries it’s just frustrating as we leave the rivals until they are level four. I must just be unlucky 
Atm I’m finding lv 4 tanks have a better drop rate for experimental items then any other lv 4 bots, I’m on xbox and don’t know if that’s the same for other platforms
I’m playing on the PS4, so sounds like the XBOX is luckier out of the two consoles. 
Keep working lv4 tanks (fnix and apocalypse) I got my exp Kvm 59 from one of those today
I got my experimental Kvm from a level 4 Military Tank.
Moved to Feedback/Feature Requests 
/Mod
And i got my 6* KVM 59 from Forest region FNIX harv rival (lvl4). It was the 7th rival i iced since March update. Also, i play solo with Guerilla difficulty.
So, going after only tanks may not be the way. Out of the 7x i iced, only one was tank, 2x were hunters and rest were harvs. All rivals were also lvl4, except one mil hunter who was lvl3.
Drop rate has been increased for sure, four exp. drops after patch for me.
I just finished clearing my map of 64 lvl 4 rivals and of that I got 7 experimental weapons:
Machines class and type doesn’t seem to matter. I got X-PVG90 from a prototype Hunter and apocalypse harvester.
I think I killed like 14, maybe 16 rivals… nothing…
Then 4 in a row gave me something.
It’s quite interesting, really…
I LOVE it…
Drop her at your broken! I’ve killed 4 rivals, different levels, different types, got the same brown experimental gloves.
I just got the Experimental Granatgevär m/49 takes two rounds and takes about 10% of the tanks. should have come with a better sight. I been playing from the start and have 6 experimental guns,
Automalevar 4, pansarvarnsgever 90, M/46 Kpist SMG, Sjoqvist semi auto shot , Klaucke 17 Granatgevär m/49. and still looking. Anyone know if there’s a AI-76 Experimental, I like this gun it awesome with the extended barrel, and does a lot of damage to the machines so a Experimental would be real handy rifle.
I found that the Meusser Hunting rifle .243, is better than the Aligstudsare Hunting rifle .270
Hint to the developers Watch Kentucky ballistics on your tube .50 cal brass rounds, and the Elephant Gun single and double barrel, maybe the 10 bore shot gun.
do like reading the comments, and learning things about the game.
Actually the Aligstudsare Hunting rifle is better than the Meusser, it makes more damage, the problem is it has a smaller mag than the other weapon.
If you don´t believe me, try shooting hunters and runners with both weapons.
For a list of the weapons check the wiki(fastest way).