Hi Ian. lucky you I did the very first one,
O.K. this is how it works,the reason the stops are removed and the slot extented, is to allow the gears to disengage,Yhe motor runs backwards compared to normal,so when the motor is engaged it moves the gun forward just a little(almost zero)the gears disengage and the barrel is pulled back with the spring,the motor then moves the barrel forward till it hits the microswitch(you will have to fit one can be bought from maplins) and stops.
See link here to my post on RCU it should help if you need more info let me know. http://www.rcuniverse.com/forum/m_80453 ... tm#8048837