Some pre-WW II AP Cores were 4% TUNGSTEN-STEEL ALLOY, NOT
"Tungsten Carbide", which is a Pure compound ( Tungsten and Carbon), and very brittle.
During WW II they switched to a Molybdenum Steel alloy and Hardened.(Cheaper).
RA .300Z 41 etc. Contract with British Gov’t for ammo to supply Original US made Fighters and some early B17s bought before Lend lease; Supply continued under Lend-lease Act after 1941.
The .300Z is the British method of indicating .30cal for Browning Aircraft Guns(AN-M2 design) and later, M17/M1903 Rifle Ammo as well.
“Z” indicates Nitro cellulose Load, as distinct from “Cordite”.
The Round channeled Indent in the headstamp is typical of US "AN-M2 Loading of Ammo for Aircraft MG use ( as well as extra stab crimps for the primer, the annular groove improved primer sealing during firing, and also increased head hardness as well. Idea came from both French ( 8mm Lebel groove) and Italian
(6,5 cartridges) where the groove was headed to improve primer seal ( thew french to allow Tube Loading as well); all Pre WW I ideas.
The US adopted this “annular Groove” in 1917-18 with Aircraft ammo for the Marlin M1918 MG (modifed Colt 1895/14 design) to avoid primer cup extrusion Jams. The USAAF continued the use of this system for “AN-M2” cartridges into WWII, but I don’t know when it was discontinued ( by 1942, very few US Aircraft still had .30 Brownings, having “Up-Gunned” to .50 cal almost universally.
Remington kept making the AN-M2 type for Britain till at least 1943.
After WW II, most of remaining British stocks of .300Z was offloaded to either Allied nations or “the Colonies” for ground use…it was belted both in Cloth belts and steel Link.
( owner of some "British-then Dutch-then Indonesian RA .300Z )