The front piece on a real M60 is welded to the gun's gas tube.
The attached pictures are of the M60D (aircraft version) because it doesn't have hand guards, so you can see what we are talking about. Basically the same setup.
In the second picture, the part marked A is attached to the front post of the mount, on the left side of the mount in 67beast's first photo. The gun then rotates down and the pin marked B clips into the two spring loaded claws. (Clear as mud??
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The same method is used to attach the gun to the M142 mount.
A buddy of mine made me a front mount by welding two vertical ears to a small curved piece that attached to the gas tube (the lower tubular part under the barrel) with hose clamps. It has a small piece of rod welded between the ears to attach to the mount. Works like a charm.
If your gun doesn't have the pin at the front of the receiver, just the holes, you can get the right sized rod at Home Depot. I had to add it to one of my guns. I drilled it for pins so it doesn't slip out.
Cheers