You've confused me now..... 
 
alias backtorifle "useweaponclass rifle" 
alias backtosmg "useweaponclass smg" 
alias backtomg "useweaponclass mg" 
alias backtoheavy "useweaponclass heavy" 
bind DEL "useweaponclass grenade" 
bind KP_2 "backtorifle; bind PGDN backtorifle" 
bind KP_3 "backtosmg; bind PGDN backtosmg" 
bind KP_4 "backtomg; bind PGDN backtoMG" 
bind KP_6 "backtoheavy; bind PGDN backtoheavy"
Trying to use above this script..... where does:
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alias backtorifle "useweaponclass rifle; bind PGDN riflegren; bind PGDN riflesmoke" 
alias backtosmg "useweaponclass smg; bind PGDN smggrengren; bind PGDN smgsmoke" 
alias backtomg "useweaponclass mg; bind PGDN mggren; bind PGDN mgsmoke" 
alias backtoheavy "useweaponclass heavy; bind PGDN heavygren; bind PGDN heavysmoke" 
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come into it?
what exactly does KP_x do?
Thanks
Rimmel