While you are on the site, in your browser find and click "Add to Home Screen" A CGN beaver app icon will then be created to your phone that is directly link to the site.
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That auction ad selling replicas claims that they manufacture the extensions themselves to look like what was used in the movie (it's not a real product otherwise as far as I can tell). Maybe you could convince them to build and sell you just the prop extension?
Personally, it never really looked like it belonged, even on an over-the-top gun like a .50 AE Desert Eagle