These flechette packages were developed by/for Picatinny Arsenal. The 222 Rem Mag loads pre-date the 5.56 x 45. The Rem Mag cases were primarily in sabot material development. I suppose these larger cases were utilized in order to maintain enough case capacity. The grey material sabot was developed and finalized. There are dozens of variations of sabot materials and designs. Hand-in-hand with the sabot development was the development of the boot (the little cup-like piece that kept the rear end of the sabot in contact with the shaft of the flechette. These are all ‘puller’ type sabot designs (as opposed to ‘pusher’ type)). Ultimately, the final flechette/sabot package ended up being used in the ACR trials (Advanced Combat Rifle) after a technology transfer to Steyr of Austria where it was further developed and presented in their telescoped ‘SCF’ – Synthetic Cased Flechette round. There are no official designations for these rounds. BTW, I’ve never seen a 1979 round loaded with the grey sabot package before.
These are .222 Rem Mag cased rounds.
These are 5.56 x 45 cased rounds.