There’s a lot of ways to do this, we would need to see how you switch weapons to get a really specific answer. But here is my best guess:
On your bag blueprint, create an array variable of Magazine object references
Every time you spawn a magazine, add it to the array variable
Every time you switch a magazine, somehow get the index of the magazine you are switching to, then use that index to get the reference from your array variable. Then you will have access to whatever variables are on your magazine, like current bullets.
If you don’t know how to get the index, I will need to see a screenshot of how you switch weapons