As  part of a joint NATO program involving many countries working on different ideas
to find a cartridge of less than .223" diameter or 5.56mm caliber, the U.S.A.
experimented with many different size cartridges in the 1960s to mid 1970's.
The 4.32x45mm (.17"Cal.) (XM16E1)* and/or (FA T211)* experimental prototype
by the Frankford Arsenal was used in rebarreled M-16s and SPIW
(Special Purpose Individual Weapons) see picture below.
Basically a 5.56x45mm (.223 cal.) necked down to 4.32x45 or 46mm or (.17cal.), it was
abandoned after the U.S.A. decided to stay with the 5.56x45mm.
*facts from different sources,
$ 14.95 per Cartridge
4.32x45 mm
Experimental Prototype
Cartridge Comparition
1973 (FA73) H/S
4.32x 45mm