I'm considering ordering Uncle Mike's "Butt Stock Shell Holder With Flap" for shotguns. How well do they fit the stock of standard shotguns such as my Beretta A390? Do they slide about on the stock or do they fit tightly? Can it be adjusted to fit different stock sizes? Here's a link to a piture of it: http://www.unclemikes.com/m_shooting_p.asp?grpky=83