Relatively simple. In config.cpp, search for
class M16 : Riffle
...
...
class burst
And change the following:
burst=3; <- change to 1
sound[]={"\WST_Weapons\Sounds\M16Burst.wss","db0",1}; <- change to M16Single.wss
autoFire=0; <- change to 1
Voila, fully auto M16. You may want to either change the displayname to "M16 Auto", or edit stringtable.csv.