Some information on a 20mm caseless is mentioned in:
COMBUSTIBLE AMMUNITION FOR SMALL ARMS, DEVELOPMENT OF SELF-CONTAINED PROPELLANT CHARGE , Frankford Arsenal, May, 1960, pages 1 & 2
"Research on development of combustible cartridge cases for small arms was conducted in Germany during World War II. Subsequent to the end of the war, work on combustible cased ammunition for small arms was conducted in Spain by the Belgian firm of SIDEM International under contract to the European Office of the U. S. Air Force Research and Development Command (2). Both 7.92 mm and 20 mm combustible ammunition , capable of being fired single shot in modified test actions, were developed under this effort. The 7.92 mm combustible cartridge consisted essentially of a molded case with ribbed inner walls composed of ball propellant and a mixed cellulose ester (of acetic and butyric acids) binder. The 20 mm cartridge case was similar except that the inner wall of the case was smooth. These cases were loaded with the amount and type of loose granular propellant required to meet the desired ballistic performance. The primers used in these cartridges were also combustible. Since elimination of the metal cartridge case introduces the problem of obturation (sealing the chamber against loss of combustion gas), conventional small arms test actions were modified to obtain satisfactory obturation with the combustible rounds.
Tests were conducted in the United States with both the 7.92 mm and 20 mm combustible cartridges. The 7.92 mm round yielded low mean velocity (with individual shots exhibiting considerable velocity dispersion) as well as poor accuracy. In addition, the cartridge cases were not completely consumed, with propellant residue found in the test action after firing. The poor accuracy was attributed by SIDEM to notches cut into the bullet to secure proper adhesion between the projectile and the combustible cartridge case (3). The 20 mm combustible round (designed to be fired from a modified M24 test action and to duplicate the performance of the 20 mm M99 metal-cased round) was found to be generally unsatisfactory (4). At 70° F, the mean velocity obtained with the combustible round was approximately 120 ft/sec lower than that yielded by the standard M99 round. At temperatures other than 70° F (viz., 130°, 0°, -30°, and -65°F), 58 percent of the combustible rounds misfired. A considerable portion of the case was found to be unconsumed during the ballistic cycle. In addition, the combustible case was found to be very fragile, one round shattering after being dropped from a height of 30 inches. This development has since been terminated by the U. S. Air Force.
(2) SIDEM International, “Development of a Self-Consuming Cartridge,” Contract AF 61(514)-745C, Final Report, April 1957."