Shadow of the Tomb Raider
99 Achievements
1,690
47-64h
On the Go
Craft special ammunition 50 times
10
10.89%
How to unlock the On the Go achievement in Shadow of the Tomb Raider - Definitive Guide
There are a couple of peculiar things about the way this achievement is tracked. The first thing to know is that all of your 50 special ammos must apparently be crafted in the same save file. I got to 96% with this achievement on my first game, then started a new game on a new save file and the tracker never moved. I started a third game, and it still didn't move. I went back to my original game, crafted 2 more special ammos, and the tracker moved and the achievement unlocked.
The second thing is, I've noticed that many of the cumulative achievements for this game don't track when you're replaying from a checkpoint after a death in Deadly Obsession difficulty. You can still get the achievement, but it will take longer because only the special ammo that you craft on your first playthrough of each part will count. I cannot confirm that this specific achievement works this way, but so many of the other ones do, I assume this one does also.
So you need to craft all 50 in the same playthrough, but you do not need to craft all 50 in the same save file.
Like you said, the other cumulative action achievements work this way. If you revert to an earlier save, or start a new playthrough you will lose some or all of your progress.
Crafting special ammunition is as simple as holding with any weapon that you have equipped. There are a few variants of special ammunition, though the most basic one you will have access to from the start of the game is the fire arrow (which requires certain resources, such as cloth and oil). The Jaguar's Fear and Viper's Lure skills will also allow you to craft fear and lure arrows, respectively.
Additionally, you can also craft flare rounds and concussion rounds for the pistol and shotgun, respectively, by unlocking the Serpent's Glint and Boa's Blow skills.
All in all, so long as you have the appropriate skills and resources available, this achievement can be unlocked easily over the course of your playthrough without much effort. Please note, however, that you cannot hold more than three or four of a single special ammunition type, so you will need to actually use it in order to make room to craft more.