Noticed something interesting in this new Japanese Iphone Metal Gear Solid: Social Ops screencap-
It's got all the weird features of the Ground Zeroes rifle and is a really close match for the FN FAL.
My best guess is that the Ground Zeroes rifle is a modified FAL.
Also found this image with a few variations on the rifle-
And it looks like there is a version that has a magazine well that isn't slanted like most rifles.
Now looking at this picture it looks like the magwell is slanted on one side and flat on the other aid in slapping in new magazines faster.
I'm guess the features that are off from the real deal come from simplifying the modeling. Like this guy did with the FAL-

It's got all the weird features of the Ground Zeroes rifle and is a really close match for the FN FAL.

My best guess is that the Ground Zeroes rifle is a modified FAL.
Also found this image with a few variations on the rifle-


And it looks like there is a version that has a magazine well that isn't slanted like most rifles.

Now looking at this picture it looks like the magwell is slanted on one side and flat on the other aid in slapping in new magazines faster.

I'm guess the features that are off from the real deal come from simplifying the modeling. Like this guy did with the FAL-
