I.B. 4-lb. Mks IV, IVE, V, VE, and 4-lb. "X" with 2- and 4-minute delays, Mk II (Service)

Fuzing

Color markings

Over-all length

Width across flats

Total weight

Description: The Bomb Mk IV is hexagonal in shape, having a hollow magnesium-alloy body, and a cast iron nose, with the body cast to the nose in manufacture. A tin-place tail, closed by a tail cover, is secured to the other end of the body by three drive-screws, which also hold a steel striker housing in position in the body.

At the tail end, the body is counter-bored to accommodate an ignitiong mechanism consisting of a striker, located in the striker housing, and a steel plate seated at the bottom of the counterbore and containing a 1.7-grain detonator. One end of the striker is pointed, and the other end projects through a hole in the striker housing. The edge of this hole is chamfered to permit a thin brass cross, secured to the striker, to be bent and pulled through the hole when the bomb functions. This cross forms a striker support dur-ing transit and storage. Movement of the striker forwards the detonator is, when the bomb is packed in its case, prevented by a spring-loaded safety plunger housed in a sleeve. Two vent holes, plugged by cork inserts, are provided in the tail end of the body and communicate with the space between the igniter mechanism and the bomb main filling. A strip of primed cambric is located in this space. The Mk IVE is identical to the Mk IV except for a black-powder burster charge, just abaft the nose plug in the body cavity, which replaces a small quantity of the thermite filler.

Functioning: When the bomb is released from the container, the safety plunger springs out, and, on impact with the target, the striker moves down, breaking free from the brass cross striker support and firing the detonator. The flash from the detonator ig-nites the primed cambric strip and the priming paste (gunpowder-shellac paste), and the products of combustion blow the cork inserts out of the ventholes. The paste ignites the pressed priming composition which, in turn, ignites the thermite pellets. The magnesium-ally body starts to melt about 25 sec. after the bomb has ignited, and burns for about 10 minutes. In the Mk IVE, after 1½ to 4 minutes, the gunpowder in the burster is ignited and explodes.

Suspension: These incendiaries are carried in the Small Bomb Container or in Cluster Projectiles.

Filling: The bombs are filled with thermite incendiary pellets and the priming pellets which ignite them. The incendiary material from which the bodies are manufactured, a magnesium alloy, also contributes to the incendiary effect. The "E" bombs have an added black-powder burster charge, while the "X" bombs have a C.E. burster.

Remarks: The 4-lb. Mk V and VE are similar to the Mk IV and IVE, except for the fol-lowing differences. The bomb is inidiated by a cap and anvil being struck by the striker, the striker having a blunt point. The cap is supported by an aluminum plate, and the striker housing is made of aluminum. A different kind of pressed priming composition is used, which is more violent than the priming compositions used in the Bomb Mk IV, and cuses some of the magnesium-alloy to scatter when the bombs are ignited. This bomb is actually the U.S. AN-M50.

The 4-lb. X, with 2- and 4-minute delays, Mk II is similar in construction to the Mk IV, except that a C.E. exploder is contained in the steel nose; this exploder being initiated after delay of either two of four minutes. The nose is painted dull red, for 3½ in., with two ½-in. black bands separated by a 1-in. bright red band painted on the nose. A se-cond bright red band, ½ in. wide and ½ in. abaft the dull red coloring, indicates that the incendiary is of the explosive type.

No "E" marks are now produced. The small black-powder charge was not lethal, but served only as a detterent.

Figure 39 – I.B. 4-lb. Mk IV

Figure 40 – I.B. 4-lb. "X" with 2-minute delay, Mk II