.
.55 Boys Anti-tank
.
British Military Cartridge
.
It became evident in World War I that tanks were going
to be a big problem and so it was decided that "Anti Tank
Guns" were needed to stop the big metal monsters.  The
first anti-tank cartridge was the 13.2x92mm for the Mauser
style bolt action,Abwehr Gewehr M1918 Rifle,  during
World War I.
The 55 Boys "Anti Tank" Rifle was developed by the
Assistant Superintendent of Design for the British Small
Arms Committee, Captain H.C. Boys and was named in his
honor after he died in 1937, just before the rifle was put
into service for the United Kingdom.
It became obvious in the early part of World War II that
the "Anti-Tank" rifle era was going to be short lived, due
to the weight and manpower needed to use it on the
battlefield. As soon as World War II was underway
"Anti-Tank" rifles were replaced with rocket powered
devices.
This is one of the largest "belted" cartridge that was ever
produced;  Only made by Kynoch and RG (Royal
Ordnance) the .55 Boys or 13.9x99mm is a huge cartridge
with a (over 900 grain) steel armor piecing projectile that
can penetrate
up to 3/4 inch (20 mm)of armour plate steel at 100 yards.
Long Obsolete.
On the above Photos, picture on the right side: for size
comparision, on the left 55 Boys, on the right .223 or
5.56x45mm NATO,  Below picture , Boys Anti-Tank rifle
.