To add flying brass, open this pic in PS along with your image.
I found the 5.56 casing on a stock photo website, and edited it for ease of use.
Using the Magic Wand tool, select the white space around the casing.
Then click Select>Inverse to select the actual casing.
Copy/Paste it onto your image, or you can also hold down Ctrl and drag the layer onto it.
Now, using Edit>Transform>Scale (as described in Step 3 above), you can resize the casing, holding Shift to maintain aspect ratio. Use Edit>Transform>Rotate to rotate the casing (obviously).
When you’re happy with it, click anywhere on the Tools pane on the left to apply the transformations.
Once that’s done, hold the Ctrl key and drag the layer to the desired position.