Ah the Australian Navy Clearance Divers carry the Australian F1 Sub Machine gun as standard (along with the M4 and Mosberg shotgun).
It still gets used because it is so simple and light. It is basically a sten gun, just with a solid stock and top mount magazine. You pretty much fill it full of sand, mud, glue, water etc and it'll keep on firing. It's also fairly accurate and can easily be changed over to have a silnced barrel assembly.
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Platoon Commander, 6 Platoon, B Company
10/27th Battalion
Royal South Australia Regiment
PRO PATRIA