just do what i do,
1. go on ebay, buy a load of foam board, uk 5 sheets of A3 5mm thick costs £8 including postage.
2. find a picture of an assault rifle and scale it up so it is somewhere around 30".
3. make a skeleton for the assault rifle, the basic version from which you will add to.
4. cut out plates and stick onto the gun.
5. paint resin onto the surface to harden card, fill interior with expanding foam.
6. spray the gun matt black and paint in all the details like the metal, scratch marks etc.
7. spray matt enamel or varnish onto the gun to protect it from any bumps.
i will be making a guide soon for an assault rifle from start to finish, hopefully it will be on in a few days with pictures. if interested let me know. i will also be doing 1 for the smg pictured below, that 1 is cardboard though but will be remade in foam board