.
In the late 1960's to late 1970's, many countries worked on different ideas as part of a joint
NATO program to find a cartridge of less than .22" Diameter or 5.56mm caliber.
The 4.6 x 36 mm cartridge was one of 4 or 5  different calibers of  cartridges used by
HK for the HK36
(not to be confused with the current 4.6x30mm for the HK MP7, or with the
current HK G36 5.56mm rifle)
There is also the capillary problem with the really small bores; any water which gets into the
barrel will be difficult to dislodge.
A couple of interesting experiments were the 4.6x36 HK for the HK36 rifle  featured a
"spoon tip"' to the bullet to encourage tumbling on impact.
Two different cartridges in this caliber are available below
                          4.6x36mm (not the 4.6x30mmHK)
MilitaryExperiment Prototype
Military Cartridge Stock Section
.
.
.
.
.
.
,,