Just received an email with a plausible explanation regarding this projectile from a friend/collector:
[quote].60/.40 discarding carrier Projectile for Penetration testing
These projectiles were used by the Watertown arsenal while conducting armor penetration tests
The projectile is a special hardened rubber sabot with tungsten carbide penetrator.
The cores were cal. .40 scale models of the ogival-nosed 90mm HVAP M304 core.[/quote]
While searching the internet continuing on this info, I found publication “AD389304 - tank automotive test results” that has following paragraphs:
[quote]IDENTIFICATION: Report No. WAL 762/231-8;
Project No. TA1-5002
DATE OF REPORT: 15 December 1953
ORIGIN: Watertown Arsenal, Mass.
PURPOSE: To compare the armor penetration performance of two truncated conical-nosed and one oglval-nosed tungsten carbide cores
METHOD: The test ammunition cores were weighed and ballistically tested against 0.50, 0.77, and 1.CO-inch rolled homogeneous armor at obliquities ranging from 0° to 60°. A protection ballistic limit was obtained for each attack condition. All fragments larger than 1/20 inch in maximiün dimension were recovered, and particular emphasis was placed on identification of nose- and base fragments. Ballistic limits were recorded for each test condition and plotted as functions of plate thickness and obliquity. Perforation energies were recorded for each ballistic limit in order that a direct comparison could be made with any cal. .40 tungsten carbide core
DESCRIPTION: The test material consisted of three types of cal. .40 tungsten carbide cores manufactured by the Carboloy Company.
PURPOSE: To evaluate the performance of scale model plastic sabot projectiles fired in a cal. .60 test barrel
DESCRIPTION: The majority of test projectiles were scale model 90mm AP shot, cal. .400-inch.
Miscellaneous rounds included scale model rocke assist projectiles and cal. .45 ball ammunition.
All of the projectiles were mounted in carriers .608-inch in diameter.
The carrier material used was “Incite” except for certain rounds which were made of various plastic materials supplied by the Naval Research Laboratory.[/quote]
So i guess we could assume it was a saboted projectile designed for armor plate testing.