Currently it doesn't work with FPS arms because it expects a humanoid model.
Please make it work with FPS arms. I don't think it'll be hard. The Weapon and Ammunition objects just need to be more generic. Also, under 3D Model where you insert a gun to be attached to the hand, that isn't necessary, because the 3D Model should be parented to the Main Camera.
The following free asset is really good as it has everything from animations to sounds. It has 2 weapons, so you can set up draw and holster weapons, which I'm having trouble with. I've seen a developer use it in a tutorial video as well, so maybe you can make a video and kill two birds with one stone :) https://assetstore.unity.com/packages/3d/props/weapons/low-poly-fps-pack-free-sample-144839
For the Weapon object, I think it'll be useful to add a "On Aim" and "On Ease" action list like what you have in the Ammunition object.
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