Make sure to first have SuperBLT, and BeardLib installed. Click the links in the dependency list.

Optional video tutorial for installing SuperBLT, BeardLib, and mod_overrides mods: https://youtu.be/rin_sjSJIkg

1. After downloading the mod, use a program such as 7zip or WinRAR to extract the main folder of the mod.
2. Navigate to assets/mod_overrides. The assets folder is supposed to be in your game's installation folder (don't know where that is? click here). Create the folder if necessary
3. Move the main folder of the mod from step 1 to the folder you navigated to in step 2.

Extract the space ar15 into mod_overrides.

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To my knowledge, I didn't put anything in here that requires Weaponlib.
That is subject to change assuming that there is something Weaponlib related in there.

You do need Superblt/Beardlib for asset loading.