I had this problem too, it happens when you change to iron sights, when the crosshair lines up the models ironsights. I worked out a way to fix it though. This has only started happening when I downloaded the latest update 1.18. Now to fix this problem.
First go to this page
http://forum.thegamecreators.com/?m=forum_view&b=21&t=172443&p=0
and download the zip file that errant has provided at the bottom right of the first post.
Next go into your folder The game creators, FPS Creator, Files, Gamecore, guns, where you'll see the model pack folders.
Next open up the zip file you just downloaded, it will contain all the gunspec files for all the weapons for model packs 9 and 10, you'll only need to copy over the M16 variant gunspec files though since the rest work properly.
I checked over the gunspec file settings for the M4A1 in the zip file and compared them to the original.They were exactly the same however copy pasting over them solved the problem.
Hope this helps