Dave - the magazine for this was of the “stick” type (as opposed to the drum magazines) and held 18 rounds. It looks just like a 20 round box magazine for the Thompson except that it is deeper (from front to rear) to accomodate the longer cartridge. It fit into the magazine well of the Tommy Gun because it is not wider than the drum magazines. When a 20-shot or 30-shot box magazine is in a Thompson, there is a noticeable gap in front of it up in the magazine well. This gap is pretty much filled by the 18-round shot cartridge magazine.
These could be used single shot in a pistol, but were really intended, in their commercial form, for the Tommy Gun. The earliest Peter’s catalog I have from the era of these cartridges is 1938 (they absolutely came out before then; I simply am weak on pre-war catalogs), where the round is offered as Item No. 4525 .45 Automatic Shot, RUSTLESS" with the advice “Adapted to Thompson Sub Machine Gun only - not intended for .45 Automatic Hand Pistols.” They were, in 1938, packed 50 rounds to a box and 2,000 rounds to a case.
I have five different box labels for the commercial load, which was only offered by Peters, but I believe that there is at least a sixth known, and perhaps more. There are many variations of the commercial round, all headstamped as you show, “PETERS .45 A.C.” The earliest round had only one deep case cannelure, with all variations follwing it having two. There are variations in the height of these cannelures, and in the spacing between them. Early rounds are with copper primer cups, later changed to nickeled cups. The last production rounds had nickeled cases as well, with the final one having a blue-colored sabot rather than red.
The last catalog listing I have for this round is 1950, The next catalog I have is 1952, and it no longer lists it. Only Peters sold this round commercially.
HWS II pages 25 and 26 have total information on the military version and need not be repeated in full here. The round was officially approved, patterned after the commercial loading, August 20, 1943, as the “Cartridge, Shot, Cal. .45, T23.” In October 1943 it was standardized as the “M12.” The load was six grains of Bulseye powder and a waxed red paper bullet containing 125 to 133 size 7-1/2 chilled lead shot. They were made both by Remington Arms using the “R A” headstamp, and by Evansville Chrysler using their usual stunning array of headstamp bunters with the initials “E C” and the date. We have only seen “43” dates from Chrysler. Remington headstamps show both “42” and “43” dates.
Remington made a version of the T23 loaded to the normal length of a .45 pistol ball cartridge. HWS II reports one in a headstamp of “R A 42” although my specimen is headstamped “R A 43.” In several collections, including my own, I have seen the Remington rounds with the sabot colored silver. It appears original, with no overlap of the color onto the case. I know of no explanation for these rounds and, of course, there is always the chance that they are “fakes” (although maybe not done for the purpose of deceiving collectors, as I paid next to nothing for mine, and the same was said by at least one other collector who has one).
The military rounds were boxed 20 rounds to a carton. The original cartridges had the warning, in English, Japanese and German "Use only for hunting game. Do not use against enemy troops (Nur bei der jagd auf Wild zu benutzen! Darf nicht gegen feindliche Truppen gebraucht werden!) Advice by the U.S. Army Judge Advocate General made it unnecessary to continue the tri-lingual marking, and it was changed to English only, and read “For use in Hunting Small Game. Effective range 25 feet.”
Since this ammunition was a USAAF requirement, it is clear that it was intended primarily to used singled loaded into pistols. Not many air crewmen were armed with the the TSMG and the idea of the cartridge was as a survival round for downed air crewmen. There were instructions with the rounds for single-loading the pistol, which included how to clear a live round from the barrel, pushing it down to fall out thru the magazine well, since it was too long for the ejection slot on the M1911 and M1911A1 pistols in use.
This cartridge was replaced in the military by the all-brass M15 .45 cartridge due to unfavorable reports from the field primarily relating to the swelling and cracking of the paper sabot of the M12 in hot, humid climates. It was also found that the paper bullets did not break up consistently, causing erractic shot patterns. The length of time the commercial version was sold by Peters may indicate that under civilian police conditions, there was little complaint about the performance of the cartridge. This is only conjecture on our part, although we doubt seriously that the round would have been manufactured, especially for police, for very long if such complaints were forwarded to Peters from LE Agencies.