Design and Development of the U.S. Carbine Thirty-Round Magazine
By Frank Iannamico

The .30 caliber carbine was designed in early 1940 to provide U.S. Army and Marine Corps officers and NCO’s a lightweight semiautomatic weapon. One feature that the troops liked was the light 5.2-pound weight of the new carbine, compared to the somewhat heavy 10.5 pound M1 Garand rifle. The second feature of the carbine that was well received was its easy to load, detachable fifteen-round magazine. The M1 Garand rifle was limited by its eight round enbloc clip that could not be topped off; the clip would not eject from the rifle until it was empty. While the stopping power of the carbine’s small .30 caliber cartridge proved wanting when compared to the full power .30-06 Garand, the other features made it a popular weapon.

As the carbine was pressed into service there were soon a number of requests from the field for a select-fire feature for the weapon. The original U.S. Ordnance requirement had specified a select-fire feature but in order to get the carbine manufactured and in service as quickly as possible, the select-fire requirement was dropped. After the carbine was adopted and distributed to troops in combat, the ever diligent U.S. Ordnance Department continued its efforts to improve and enhance the carbine. One of the projects was development of a suitable full automatic feature that could be easily added to the carbine with minimal alterations to the basic design. Ordnance wanted to be able to easily upgrade existing semiautomatic M1 carbines in the field with the select-fire feature.

Early in 1944, the Inland Division began to develop a method to convert the M1 carbine into a select-fire weapon; in other words, to have both semi-automatic and full-automatic capabilities. Two Inland engineers, Paul Hamisch and Frederick Sampson, designed a conversion that required only a few new parts along with minimum changes to the weapon’s original design. The experimental select-fire M1 carbine was designated as the T4. After extensive testing of the new variation of the M1, the weapon was recommended for adoption as the Carbine Caliber .30, M2 in September of 1944 and was quickly adopted as Standard in October 1944. Subsequently, both the M1 and M1A1 carbines were reclassified as Limited Standard. By the time the weapon began getting into the hands of the troops in the field the war was almost over. Thus, the M2 carbine saw little combat use in World War II. The carbine in the M2 configuration had a cyclic rate of 700 to 750 rounds per minute. At that rate, the original fifteen-round magazine proved lacking. However, as the engineers worked on the conversion of the carbine to select-fire, others were working on perfecting a high capacity magazine to feed the weapon.


During the design and development of the .30 M1 carbine there were prototype box style double stack, double feed magazines made in ten, fifteen and twenty cartridge capacities. The fifteen-round magazine proved to be the best overall design and was adopted for use with the carbine. The fifteen-round magazines were manufactured in very large numbers by well over sixty different contractors and subcontractors.

In December of 1944, U.S. Army Major Fargo submitted two modified carbine magazines to the Infantry Board. One of the magazines had a thirty-round capacity. This magazine was constructed by welding two standard fifteen-round carbine magazines together. Because of the carbine round’s slightly tapered case, the magazines were joined together at an eleven-degree angle. The studs for holding the magazine were larger than normal and a third stud was placed on the magazine’s left side for added support for the heavier weight of the magazine. The fifteen-round magazine submitted had steel strips in place of the standard embossed studs.

The test magazines were photographed, measured and weighed. The time required to manually load each magazine was measured and recorded. It was discovered that loading two fifteen-round magazines took three seconds longer to load than the time required to load one thirty-round magazine (thirty-eight seconds). The required pressure to load the magazines by hand felt the same in both the fifteen and thirty round configurations. There were no problems encountered loading all thirty of the rounds into the experimental high capacity magazine. It was discovered that thirty-one rounds could be forced into the thirty-round magazine. Overloading the magazine increased the possibility of malfunctions to occur. The magazine weighed .26 pounds empty and 1.08 pounds loaded. The standard fifteen-round carbine magazine weighed .16 pounds empty and .56 pounds when loaded.

Three carbines were provided to test the magazines, a standard M1, a paratrooper folding stock M1A1 and a new select-fire M2 model. None of the test carbines had the new improved magazine catch that was designed to adequately support the increased weight of a loaded thirty-round magazine. The functioning of the test magazines was the same in each carbine. The magazines were easily installed and removed in all of the weapons. Abusive handling of the carbine with each test magazine in place failed to produce any damage to the magazines.

Functioning of the test magazines was identical in each of the three carbines. All of the weapons functioned without stoppages until the retaining studs embossed on the magazines were too worn to keep them intact. At this point, the bolt would override the magazine and fail to pick up fresh cartridges.

The added weight of the thirty-round magazine caused the standard magazine studs to shear off after the magazine had been loaded and fired twenty-six times. The magazine had to be held in place by hand for the carbine to continue functioning. There were no problems with the modified fifteen-round magazine. It was concluded that the thirty-round magazine was worth further study and development and the embossed studs of the magazines needed to be hardened to withstand extended automatic firing in the carbine.

GI Production Thirty-Round Carbine Magazines

After the thirty-round magazine was perfected, they became standard issue for both M1 and M2 carbines. They provided the operator with additional firepower that was unique for a shoulder fired weapon during World War II. The controversial lethality of the small carbine round was offset somewhat by the M2 carbine’s lightweight, select-fire capability and its thirty-round magazine.

All production thirty-round magazines were fitted with the T18 style follower that was designed to hold the bolt rearward after the magazine was emptied. The bolt will close when the empty magazine is removed from the weapon. To retain the bolt rearward, the pin on the slide would need to be depressed before removing the magazine. After a fresh magazine was inserted into the weapon, the slide needed only to be pulled slightly rearward and released and the carbine was ready to fire.

During World War II, the thirty-round magazines were not produced in as large of numbers as the former fifteen-round version had been. Early thirty-round magazines were manufactured with a “seamless” back, known to collectors as the “hardback.” This style magazine, made from a single piece of sheet metal, is somewhat scarce. The most common variation is the thirty-round “split back” type made from two separate pieces of sheet metal joined together. Carbine magazines were originally with a blue-black finish that varied slightly from contractor to contractor. During and after the war many thousands of magazines underwent a rebuild process in which many were refinished with a gray-green Parkerizing.

Magazine production continued during both the Korean and Vietnam Wars. All original U.S. government manufactured thirty and fifteen-round carbine magazines were marked with the contractor’s initials on the back. Original GI issue magazines are quite reliable. Some examples of the GI contractor’s markings are; IU (Underwood), AI (Autorye-Inland), SEY (Seymour), J and J (Saginaw), and OKAY (OKay Industries). Other yet unidentified markings were KI, M2, and K. There were a few magazine “accessories” produced that included plastic and rubber dust caps, stripper clips for fast reloading, and several styles of magazine carrying pouches.

Aftermarket Thirty-Round Carbine Magazines

Many of today’s carbine owners have complained that their carbines will not function properly with a thirty-round magazine. The primary reason is probably an aftermarket magazine, or the carbine in question isn’t fitted with the M2 style magazine catch that was designed to support the added weight of a thirty-round magazine.

Up until approximately ten years ago original U.S. GI thirty-round magazines were difficult to find. To fulfill the need, a number of commercial companies manufactured the magazines in quite large numbers. There have been many problems encountered with the commercially produced carbine magazines. The studs for holding the magazine in the weapon are usually not hardened, and quickly wear or shear off. Another common problem encountered is inadequate follower spring pressure and, lastly, magazine bodies that are out of spec. Aftermarket magazines usually can be identified by a lack of manufacturer’s markings, but a few companies did mark them. A few of the known aftermarket markings were; Jay Scott, and M2 (with a larger font, and stamped deeper than original GI M2 marked magazines). The commercially produced magazines were made with a blued finish, stainless steel and some were even chrome plated.

Foreign Carbine Magazines

After the end of World War II, United States began to give away thousands of M1 and M2 carbines to friendly nations as part of military aid programs. In order to maintain the carbines in service many of the receiving countries needed to manufacture replacement parts that included magazines. Most of the foreign made government contract magazines were well made and reliable. One known post World War II foreign marking was AYP on magazines made in the Netherlands.

Like many U.S. made commercial aftermarket magazines, most foreign government made examples were not marked with a manufacturer’s code. However most of these magazines can be identified by their reinforced or hardened retaining studs (hardened studs will be slightly discolored). When compared side by side with a commercially produced magazine, most foreign military magazines are of superior finish and quality and can be easily recognized.

This article first appeared in Small Arms Review V8N11 (August 2005)
and was posted online on May 3, 2013


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