These cufflinks are made from genuine British Armed Forces 5.56 bullets.
The 5.56 round is used in the British Armed Forces SA80 rifle. 
The SA80 is a British family of 5.56x45mm NATO service weapons used by the British Army.  The L85 Rifle variant has been the standard issue service rifle of the British Armed Forces since 1987, replacing the L1A1 Self-Loading Rifle.
These cufflinks come in a gift box
Bullet diameter 6mm