The 4.85×49mm is an experimental intermediate firearm cartridge made by the United Kingdom for the Individual Weapon Project, which became the SA80 series of small arms.

During the 1960s, the United Kingdom experimented with creating a lightweight but effective replacement for the 7.62×51mm NATO round.

[2] However, in the 1960s, a West German study proposed that an ideal cartridge would have a 5 mm or smaller calibre.

The results of this study encouraged the UK to research and develop bullets that were 5 mm, and smaller calibres, for their experimental cartridge.

[3] This resulted in an official decision that the round for the rifle that would become the SA80 was to be 5 mm, or slightly smaller.

However, before the production lines were fully retooled for the new round, RSAF Enfield required a batch of ammo for testing.

Existing 4.85×44mm rounds had their necks manually stretched out 5 mm in order to qualify for the test.

Lots of both the XL1E1 ball and XL2E1 tracer round were created in order to provide enough ammunition for the tests.