Gunfire, reloading, and other sound effects from this mod are not affected by Sound Physics Remastered. However, when these same sound events are triggered via the /playsound command (e.g. /playsound cgm:item.assault_rifle.fire neutral @p ~ ~ ~), they are affected by SPR as expected.

Cause (from what I can tell):

I think that the mod is playing sounds using custom or direct audio handling methods (e.g. OpenAL or server-triggered sounds) that bypass the vanilla client sound engine (SoundEngine). Sound Physics Remastered only applies acoustic effects (e.g. occlusion, reverberation) to sounds that pass through the standard sound system via registered SoundEvents on the client.