Venom,
He built that with it being both captured by the receiver and held in place by a barrel/ hand guard mount. I think I have a pic somewhere that shows the weapon with a rail, optic and suppressor. Give me until tomorrow to find and post it.
PS. Thanks for the comment about the website. All of our work we do ourselves, including the website and pics. Phillip says that we compliment each others work but I think websites are way harder than building machine guns