"I want my records to have a unique number assigned automatically, but I don't want to use the built-in autonumber."
First think through why you need to build your application that way. There are lots of arguments to be made that the autonumber is the best way to assign IDs, and that the primary key on your tables should be an autonumber. That being said, in one of my biggest databases I chose to generate the unique IDs in code instead of using an autonumber field. I was worried that if I needed to merge data between two copies of the database (for instance merging two separate projects) I would need to be able to change the ID number on the records before merging). I have still never merged data, but the database is working will with my "manual autonumber".