The rules on the web page say call ahead and advise your intent. Then complete the registration within 48 hours. I'm betting the log your phone call into a book of some sort so if you are stopped there is proof you complied. I'd always get the officers name when calling.
I also read that if you don't register your firearm, DC will not process your application.
I also read that if you don't register your firearm, DC will not process your application.