Here’s what HWS say in chapter 8 of History of U.S. Military Small Arms Ammunition, Vol III:
"…The standard cal. .50 incendiary cartridge at the end of WWII was the M23, which had generally replaced the M1 except for ground and U.S. Navy use. The M23, nicknamed the “super incendiary”, had performed well during the war with one notable exception; when fired in prolonged bursts in worn barrels, the bullet would often burst near the muzzle. In 1945, Frankford Arsenal had set up a production line for the M23, but because of technical problems, had never attained full-scale operation. However, continued development of the M23 was authorized into the post-war period, and the Arsenal started a redesign effort in 1946 to correct the premature ignition problem. This effort would continue through 1953 as Engineering Test Program #96. The initial design was designated the FAT18, and centered on increasing the strength and security of the cup inside the bullet containing the incendiary mixture. At least one test lot (FA X50-1087A) used inert-loaded M23 bullets with tinned jackets and dark blue/light blue bullet tips. Very little is known of this work except that it was apparently not completely successful.
In 1951, DRI was given a development contract by the Ordnance Department for a comprehensive study of small-caliber incendiary bullets, and as a separate task, was to recommend an improved design to replace the standard rounds then in inventory. To assist this study, Frankford Arsenal was requested to provide engineering and test support to include the providing of special test samples as required. One of the rounds provided used a case headstamped F A 53 loaded with an M1 incendiary bullet with a blackened jacket. Exactly why the bullet was blackened is not known, but it is possibly an inert loading for recovery tests. Another loading, which may have been assembled by DRI, used a blue-tipped M1 bullet with tinned jacket loaded into a case headstamped T W 5 4 with an M52A3 electric primer. The same loading using a case without head marking has also been noted. Rounds have also been examined loaded with an electric primer and the M23 bullet with dark blue /light blue tip. Reportedly this was also loaded at DRI.
In early 1951 Frankford Arsenal was ordered to start production of the M23 to support the Korean War. Testing of the FAT18 bullet had by this time progressed to a design called the FAT18E1, which was placed into limited production in late 1951 (Fig. 345). This bullet had a thicker GMCS container cup inside the bullet, and the incendiary mixture was changed from IM 11 to IM 28. Bullet weight was also increased from 512 grs to 550 grs. This round was designated the M23E1, although this model number was seldom used by the Arsenal. Production continued through 1954 on a low-rate basis, but continuing process problems finally caused this order to be cancelled, and during late 1954, production was transferred to Twin Cities Arsenal.
Apparently, the production problems at Frankford Arsenal resulted in less than satisfactory performance, and all the lots produced during the 1951-1954 period were regraded, then suspended from issue. When production of the M23E1 was transferred to Twin Cities, they used the Frankford Arsenal FAT18E1 design and imposed very tight controls on the assembly and loading of this bullet. Production started with Lot TW 18070 during November, 1954, headstamped T W 5 4 with brass primer and purple seal, and continued through March, 1955, with Lot TW 18093 headstamped T W 5. Unfortunately, the Twin Cities production of the M23E1 did not completely solve the burst bullet problem, and premature functioning continued to be reported, but on a less frequent basis. Frankford Arsenal did work on an advanced design of the M23E1 bullet called the FAT18E2 during 1955, but this effort was cancelled in early 1956 with the end of the Korean War, and increased emphasis was placed on the development of larger caliber (20-30mm) aircraft weapons and ammunition.
A round headstamped R A 52 with dark blue/light blue bullet tip, brass primer and purple seal has been examined, but there is no record that Remington made the M23 during this time period. …"
I’m working hard on the book’s format and edit, but it takes a long time to get it exactly right. Believe me, it will be worth the wait.