since my last post im back in the RA now...in 40th regt which has converted from as90 to lightgun...and believe me i thought id rejoined the infantry they are constantly in iraq or afghanistan in the infantry role...which is a bummer cause if id wanted to be hardore infantry i wouldnt of joined the RA...but nevermind....anyway to your question i asked somebody about transfers ect.....you can change posting at any time i.e moving to different units within the army...unless on operational tour....and you can change cap badge at any time...unless on operational tour...HOWEVER i was told that if for example you go for marines you have to go through the whole 32 weeks and commando course,BUT if ya fail thats you out of the forces you cant go just transfer back to the army so you will be a civvy so big risk to take.....
i advise as doc said you giving away 22 years of your life make sure you choose the right job.....but dont listen to the careers office too much as they will often try to get you to join where they need you rather than where you want to go....

