yeah i got the robot arm you unlock it at lt.colonel, buy the chest peice which unlocks at lt.colonel and it will unlock another on the same only it has the robot arm, cant remember the name but it has shotgun shells on the chest
after buying COLLAR/BREACHER it will automaticly give you the option to buy COLLAR/BREACHER [R], it's the same chest piece, but the only different is your right arm is robotic now.
*just in case your wondering, yes i do have it ^_^ *
i have it
you have to buy the chest piece that unlocks at Lt. Colonel then when you buy it the other one will unlock