You are talking about your screen name, right?
If you just try to re-register with a different name, you will get a message back saying there's already a user with your e-mail address.
What I did was to change my e-mail address to a fake one on my profile.
Then I re-registered using the correct e-mail address.
Then I sent a note to the moderators or webmaster (I forget) telling them what I did and asking them to delete the original screen name (by the way, the new user name I chose was also different).
The alternatives would be to ask a moderator or the webmaster to make the desired screen name change for you, or get a new e-mail account and register from that, though I can't encourage multiple logins at ar15.com - there's been a guy or two who have gone way too far with this.
[red][size=4] P.R.K.