In this short tutorial I explain how to rig a low poly weapon with Blender 2.90 for exporting as Unity game asset, but you can also use Blender 2.8x.
In the video I use vertex groups for rigging the bones, so that I can easily use an armature modifier with vertex groups enabled to assign the separate parts.
With this approach you can rig a model like the one in the tutorial (pistol) in about 3-4 minutes.
I am currently working on a Low Poly FPS kit for Unity, the models and animations I create with Blender.
I export the model and the animations then to Unity, add an FPS, weapons and animation controller to create the player:
The weapons will be switchable, so that you dont have to write code for this, stay tuned I will keep you informed about updates!