Pretty sure one of the devs confirmed it was all the bullets however I don't have a source rn.
Source here, level 4.
All bullets have the same slow down behavior, and always have.
This thread was added on September 30, 2019, with posts from Scriptacus.
Original Post
https://i.redd.it/ikgn4xefmrp31.jpg
How much is the slow down effect anyway? I recall Fortified legends not taking slow effects.
Correct that fortified legends do not get slowed down. The slow down hits pretty hard but wears off very quickly. There's also a few seconds of debounce where you can't be slowed again.
Does heavy ammo have any different properties to other ammo types?
Heavy ammo tends to hit harder within it's weapon class (smg, sniper, assault, etc...) but has more pronounced bullet drop and lower velocity.