Has anyone used barrel mount for laser mount........

You might try this link for something in the cheaper range, scroll down to the bottom of the page, it's for lights, but with the spacer system it might work for you.
http://www.firequest.com/catalog/shotgun_acc.html

For more expensive gadgets you can try this page
http://sidearmor.net/catalog/default.php/cPath/24

hope this helps you out.