Practice Bombs of WWII

© Jerry Penry

(Shown Approx. Actual Size)

The AN MK-5 practice bomb was one of three similar miniature practice bombs used for low-altitude horizontal or dive bombing practice during WWII. The other two were the AN MK-23 and the AN MK-43. The MK-5 has an overall length of 8.25" and is made of zinc-alloy. The diameter at its largest point is 2.18". The fins are 2.5" in length. The weight, without the signal, but with the firing pin is approximately 2 lbs-9 oz. and when armed is approximately 2 lbs-11 oz.

The main differences among the three miniature bombs are that the MK-5 is made of zinc-alloy, the MK-23 is made of an cast iron, and the MK-43 is made of lead-antimony. This gave each bomb a different weight for different purposes. All three miniature bombs used a 10-gauge shotgun shell as the signal (AN MK-4) as explained below. The MK-5 was also made to fit a different type of charge which used a small signal that contained a fluorescein dye as the filler which was activated upon striking the water. Since the MK-5 was made of a critical war metal, zinc-alloy, its supply was mostly limited to armored-deck boat targets with 1/2" deck armor. The maximum release altitude was 2,500 feet while the maximum speed at release could not exceed 290 knots. Five bombs could be carried in the MK-43 Bomb Rack and eight could be carried in the MK-47 Bomb Rack which is more fully explained below.


Stamping on a AN MK-5 practice bomb.
Many have no markings.


The AN MK-4 spotting charge consisted of an extra long 10-gauge blank shotgun shell that was inserted into the hollow chamber of the bomb. The primer end of the shotgun shell was downward so the blast would go upwards through the tail section upon impact. In the nose of the MK-5 were two small holes where a cotter pin was inserted. This pin went into a firing pin which then fired the primer on the shotgun shell. There were two slightly different firing pin assemblies that were interchangeable and functioned basically the same. The shotgun shell fit loosely in the chamber when inserted into the nose, but it could not fall out the bottom. There was a space between the primer end of the shotgun shell and the firing pin. Upon impact, the inertia caused the shotgun shell to come forward into the stationary firing pin. The other firing pin design had the inertia of the hit drive the firing pin backwards into the shotgun shell.

AN MK-4 Bomb Signal

The above drawing shows the extra long 10-gauge shotgun shell used for the signal.

From the Ordnance Pamphlet 1280 "Aircraft Bombs" 17 February 1945.
The Miniature Practice Bomb Signal AN-MK 4 is a "blank" No. 10 gauge shotgun shell (extra length). The signal has a metal base and base flange fitted to a cardboard shell. The components of the signal are: a No. 7 U.M.C. primer with an expelling charge of black powder separated from a charge of pyrotechnic (red phosphorous pyrotechnic mixture No. 7) by a kraft paper disc glued to a No. 10 gauge gun wad. The charges are sealed by a No. 10 gauge paraffined-edge felt gun wad and secured by crimping over the paper shell casing. The weight of the loaded signal is 2.1 ounces.
Note: Under no condition are these signals to be opened or tampered with. Defective signals will be returned to an ammunition depot or turned over to a Bomb Disposal Officer for proper disposal.
Signals are packed 25 to a cardboard carton. The outside dimensions of the carton are approximately 4.4 inches long, 4.4 inches wide, and 6.125 inches deep. Twenty filled cartons are packed in a wooden packing box of the type used for packing commercial No. 10 gauge shotgun shells. The outside dimensions of the packing box are approximately 25.5 inches long, 14.25 inches wide, and 10.75 inches deep. Miniature Practice Bomb Signals AN MK 4 will be stowed in approved storage facilities. The temperature of the storage facility shall not exceed 100F., and the signals shall be protected from direct sunlight, spray, or moisture and shall not be subject to excessive motion or vibration.


Extra long 10-gauge shotgun shells used for the MK-5 practice bomb.


MK-5 Firing Pin Assemblies

The two different firing pin assemblies for the MK-5 Practice Bomb. The diagram on the left shows the two steel cup construction, while the one on the right shows the one-piece lead assembly.

Red = Firing Pin
Blue = Shotgun Shell
Yellow = Cotter Pin

Two types of firing pin assemblies were used for the 3-lb., 4.5-lb. and 13-lb. miniature practice bombs. The first type was constructed of two stamped steel cups, a firing pin, and one or two spacer washers. The second type was a one-piece construction composed of cast lead. The two firing pin assemblies were dimensionally and functionally interchangeable so that either type could be used.

Both types of firing pin assemblies functioned as a result of force, but the manner in which the assemblies reacted was different. One was the result of the MK-4 signal (10-gauge shell) sliding forward upon impact, or as a result of the firing pin assembly being driven backward as a result of striking water or land. The MK-4 signal was free sliding within the miniature bomb. The cotter pin in the nose of the bomb stopped the forward action of both the signal and the firing pin assembly causing contact with each other. The cotter pin was also soft enough to be bent backward into the firing pin assembly at the same time as it was sliding toward the cotter pin. In either instance, it was almost a surity that the firing pin would strike the primer of the 10-gauge shotgun shell. The cast lead firing pin assembly achieved the same result upon impact but in a slightly different manner. The force of the impact caused the thin section of lead around the base of the firing pinto shear which then permitted the firing pin head to contact the primer on the signal. The weight of the first type of assembly was about 0.6 ounce, while the lead assembly weighed 3.1 ounces. This difference in assemblies caused the miniature bombs to have overall different weights depending upon the type of assembly used.

The above images show the two-piece firing pin mechanism. The nose of the bomb where the firing pin is inserted is milled 15/16" diameter for a depth of 1-7/16". The barrel of the remainder of the bomb where the 10-gauge signal is located is milled approximately 27/32" diameter. The cup of the firing pin that rests against the primer of the shotgun shell is made of aluminum while the end toward the nose is made of steel. A steel firing pin sticks through the aluminum cup. Between the two cups is a steel spacer. The length of both cups with the spacer is 3/4".


The MK-47 miniature practice bomb rack used for dive bombing was a long cigar-shaped metal box which held eight miniature bombs in separate compartments. It released the bombs one at a time by electronic controls of the pilot. Each bomb was thrown clear of the plane by a spring inside the compartment.

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