I'm not sure I agree with either example you gave there...we used to call our rifles 'SLR's', not rifles...and if I'm going out in my car (it's a Abarth) the colloquial term for it, at least in the Abarth community, would be "I'm off out in the handbag"! Non-Abarth owners are of course not allowed to call our cars handbags!
SMG's were...SMG's but GPMG's were gimpies; tanks were usually 'panzers' (although please don't ever call a Challenger a Chally as I never ever heard that term!!)...and of course there is the great story about the Army, after the Falklands War, calling Falkland islanders 'stills'
In my experience the Army would develop its own slang based upon random ideas and/or foreign languages - everything in BAOR was alles uber the platz, but was also buckshee, but sometimes you had to ask wasstie cosstie, especially if you were doing your dhobi...
And so on!
Even if you are one in a million like your mum told you, that means there are 7500 people just like you in the world