It may have just been me but I found I was struggling with Probuilders controls (I think I am more accustom to Blenders workflow). I decided to model my ammo props in Blender and texture them in ArmorPaint (a layer-based texturing program that has very much the same node setups as Blender but can paint on separate layers like Photoshop).
Ammo Concept art (mainly for drawing practice since they are very simple shapes anyway).
Ammo Models in Blender (UV’s all mapped onto one texture space).
Exported models in ArmorPaint (Painted all together so that they all end up onto one texture).
Final props in Unity.
Probably something I spent too much time doing but worth it to find out some sort of work flow to making assets.