heres a tutorial using my method
body
draw two basic outlines(minus the scope and barrels) from cardboard and trim them so that they are exactly the same, then cut cardboard stips to go in between the two original pieces,until you have made its basic 3 dimensional shape.
clip
cut out the visible part of the clip from the back of the rifle cut outs (before you add strips) !KEEP THESE NEW CLIP CUTOUTS! then attach a strip of cardboard to the bottom of the clip cutouts to mak it flush with the rest of the gun. to finish the clip you need to make a box that fits snuggly inbetween those cutouts, then attach it to the base of the clip.
details
you then make the two more strips in the shape of the handrest in the front and glue/tape those onto each side. after this is done, use thinner cardboard to add extra details.
barrel
make a tube out of cardstock at your desired barrel length and width, fill the barrel with either hot glue or cardboard to strengthen it and then glue to the gun, repeat for the tube under it.
scope
once the rest of the gun is finished, the last step is the scope, for the scope you can use a real scope or make one by making a cardstock tube stuck through a small cardboard cube with two sides missing
after all this is done you should have a pretty decent cardboard gun
to finish it you can strenthen it using paper mache or resin before painting, but these steps are both optional
Edit: dammit! i just spent so long making this tu and as soon as i post it i see some one gave a link to a tut