bash (or other shell) help with usernames

Trying to do something simple, but it's not working.
Just trying to pass an argument (in this case a username), and have it
touch a file in that user's directory.
calling the script like this:
SCRIPTNAME  USERNAME

#!/bin/bash
touch ~$1/FILENAME

Does not work.  I get no such directory or file.  If I hard code in:
touch  ~USERNAME/FILENAME

Then it works.
If I call it with:
SCRIPTNAME   ~USERNAME    (Note the ~)

There must be something going on with passing in the username via the
command prompt and using that string with tilde expansion.

I'm trying to do something when a user connects to an SMB share on
linux.  I can retrieve the username via $U in the pre-exec area.  I
could add the full path in SMB.conf, but not all users are in the same
path for their home directories.   Using the tilde with their username
is best.

--

-- 
Roger