Daymare: 1998

Edit: Apparently this can only be done in classic 90s mode.

There are 3 different methods of reloading in Daymare: 1998. Firstly you can press to quick reload, you can also hold to slow reload, and you can manually reload in the inventory by combining ammo boxes with weapons. The achievement is for doing the first option 100 times. This can only be obtained with guns using magazines such as the handgun, submachine gun and Desert Eagle. This does not work with the shotgun or revolver.

You can grind this out by having the following in your inventory:

1 Magazine with at least 1 bullet missing
Weapon that uses said magazine with at least 1 bullet missing

The steps:

1) Tap to quick reload
2) Pick up dropped magazine
3) Repeat 99 more times.