I would be mighty surprised if shell casings weren't included. I would mighty prefer to have adjustable settings for shell casing-stay and/or amount. Another nice addition would be to have different sounds depending on caliber and surface. For simplicity's sake, one set for pistol/SMG-ammunition, one for rifle ammunition, and one unique set for the mighty PTRS/PTRD. Grass would be borderline silent; concrete, tile and rock could share their sound, and wooden floors would have their characteristic clattering.
That would be no more than a quarter of an hour's work for a sound-engineer, although coding it would probably take longer.