All the .50BMG found in Australia is from WW II American USAAF and US Navy airstrips, depots and Army Camps.
I have found FA pre-WW II, ( 39,40 etc) as well as almost every year in the 41-44 period, and a wide range of makers. (FA, LC, RA, U, SL, DEN, etc.)
coming from North Qld and the nearby Pacific Islands.
Prior to Dec. 1941, there was No need for .50BMG in Australia, as we didn’t have any .50 cal equipped aircraft ( US Lend Lease and Direct Purchase by Britain only…The Brits had some early model B17s, and some Fighters equipped with .50s, hence the supply there.)
After January 1942, The Flood of US forces ( from Java, and direct from the USA,) saw the use of .50 ammo increase exponentially in Australia, mostly for Airforce Use ( lend-lease Aircraft, such as B24 Liberators, etc). The RAN used mostly 20mm Oerlikon in the AA role; Unlike the USN, which had .50s and also 20mm Hispanos as deck guns ( as well as Oerlikons).
The Standard RAAF Armourers Manual for 1942 carried detailed descriptions of the US .50 M2 Gun for both Turret and Fixed Mountings.
Most of my WW II .50 cal headstamps are from “Burnt” ammo ( old airfields etc cleared by Burning off, thus setting off any ammo still to be found…primers are unstruck, but case necks may be distorted or missing; occasionally a projectile is still found with the Burnt-out case; whilst the Oerlikon AA Cases are usually “once Fired” examples.
60-70 years of exposure has also corroded the Brass cases, but with a Vinegar Bath clean, and steel wool, the Headstamps become readable.
Down Under in the Deep North (Queensland)