The FAMAS and the P90 are types of Bullpup guns and after googleing and you tubeing Bullpup guns... I think a shell can be prepared for any Nerf Gun to make it more ergonomic.
I have the design's working over in my head Utilizing Plexiglass (or what ever you guys refer to it as). I bring this up though to see if any one else had any similar ideas and if so possibily helping each other in the design(s).

FAMAS

AK-47 in Bullpup form

Some gun...

And the you tube video that helped me understand how a Conversion could be possible. look into 4:50 if you want to skip ahead to understand my inspiration.