Semi-auto Knex Assault Rifle (S.A.K.A.R.)

This is just a CONCEPT, so i dont know if i will post :( it is the only knex rifle with this mech (that i have seen) i hope you can Finnish it..... it shoots two shots semi auto then you have to cock. it has a simple body and structure, it works like a real Semi-auto.... in a real gun the firing pin goes off sending force to the bullet when it explodes the bullet is sent flying. because there was an explosion there is expanding gas which has to go some were.... and so there is a channel off the barrel that turns a round and goes back to the firing pin..... so the well bullet is sent at high speed to the target. the gases are turned around pushing back the firing pin..... and then it starts again....... so i was thinking why has no one made a gun like this??? my gun works same as all knex guns do, true trigger and firing-pin but after the firing pin hits the bullet (small white rod in a hopper) the firing pin hits a ram rod which is triggerd sending the firing pin back (cocking it)..... but i dont know how the ram rod is going to be pushed back :( please help!!! comment or inbox!!!!
P.S. i put a hopper in because there was to many mechs in the way :(

Step 1: =D

Sorry Slide show coming soon (maybe)     =/