I think you're trying to fit way too much into one table.
When I try to analyze your table tStock, I come to the following.
A "projectile" has a caliber. Seems clear to me.
A "brass" also has 1 caliber. Also natural.
A "powder" can apparently be used for one or more calibers. As mentioned before, you have to put the calibers of a powder in a separate table.
Apparently "primers" can also be used for one or more calibers.
When designing your database, it is important to consider what you are going to use it for. On the one hand, this can be keeping up with the stock (of brasses, powders, primers and projectiles). On the other hand, you can record the coherence between those different components.
I have the impression that those two things are now intertwined. For example: the stock of a powder has nothing to do with what it can be used for. So you have to model these two aspects separately. So you will first have to determine the purpose of your database and then put your design in order.