There is also some confusion on how null values work in cheat engine. That I can clear up.
Null value in cheat engine can be found by changing the value type to 'string'. Same process after that.
Look for the number value, in this case bullets(30).
Fire a shot. Search for new value(29), again as a TEXT STRING.
Fire a shot. Search for new value(28), etc.
Took me three shots to find it.
Again, same process you are just looking for a text sting and not a 4-bit, 8-bit, or floating value.
Hope this helps clear up how cheat engine uses null values.