Parenting after divorce question: Should I die on this hill?

While we were married, I would never allow my boys’ father buy them bb guns. My opposition isn’t as much to toy guns in general (although I am not a fan of those either), but to the fact that bb guns are dangerous. I don’t want my kids to hurt himself or someone else, or an animal, and it just doesn’t feel right to let him have a gun. Well my oldest son is going to be 11 on Monday and my younger son confessed to me that his dad is planning on buying him a bb gun behind my back and storing it at his house. I am inclined to fight this battle, but I’m wondering what The Collective thinks.