The rail rifle is “still in the game” because you’re playing the stable version of the game - which only gets updated around all 6 months (give or take). If you want the “cutting edge” versions (including all the new cool stuff and changes, but also usually a lot of bugs), you might want to take a look at the experimental versions.
The drop-in auto sear is still in the game, even in experimental, by the way, but atomic cars are not (they got moved to a mod, last I know).
With “it’s long gone” kevin.granade means it got removed from the game on the 16. of April this year and is out of the game since experimental build 10535 as far as I can see and tell.
Eating bullets is a quality of Hulks on their own, less of the kevlar part on them.
There’s a topic discussing the spawn rate of Kelvar Hulks in which their designer explains some base ideas s-/he had in mind while creating them.