The cartridge is used in the 4 cm Flak 28 A.A. gun of the Bofors type. The length of the complete round is 17.7 inches and its weight is approximately 4 lb. 4 oz. The shell is painted yellow and stencilled in red, and has a red band painted above the driving band. The stencilling on the shell body, above the red band, includes the numeral "1" indicating the nature of filling. The shell is fitted with a single copper driving band.

The Shell H.E. filled T.N.T. and tracer composition;

Self-destroying detonator;

Tracer igniter;

Fuze Kz 38 with gaine;

Brase case or steel case coated with brass;

Propellant charge of double base propellant with a nitrocellulose igni-ter;

Primer percussion C/33 St.

The shell is substantially the original 1938 Bofors shell, modified to give a greater certain-ty of detonation and a longer range before self-destruction. Externally, it differs in that it tapers more sharply in its streamlined base. The weight of the shell, filled and fuzed is 2 lb. 1 oz. 1 dr. and its overall length when fuzed is 7 inches. A cannelure is formed rear of the driving band to enable the shell to be attached to the cartridge case. The body, at about its shoulder, is screwthreaded internally to receive an adapter, which tapers to form the forward end of the shell body and to receive the gaine and fuze.

Internally the shell has two compartments containing the main H.E. filling and the tracer composition respectibely, and a recess in the base containing the tracer igniter all of which are interconnected. The bursting charge, in the forward com-partment forming the front half of the shell body, consists of a 1 oz. 5 1/2 dr. pellet of pressed biscuit T.N.T. contained in a varnished carton with a cavity in the base to receive the self-destroying detonator and another in the forward end to receive the gaine. The cavity in the base is fitted with a brass liner.

At the base of the bursting charge is a steel plate with a central flash hole, which fits against an internal shoulder formed by the reduced diameter of the shell cavity and se-parates the bursting charge from the tracer composition. Between the steel plate and the bursting charge is a box cloth washer.

The self-destroying detonator consists of 8.5 grains of CE over 6 grains of lead azide/ lead styphnate (63/37) contained in an aluminium cylinder with a flash hole closed by a fabric disc in the base. The forward end of the cylinder is closed by an aluminium washer retained in position by turning over the mouth of the cylinder.

The tracer composition pressed directly into a compartment of slightly less diameter and length than that containing the bursting charge, is a puplish composition weighinh 1 oz. 2 1/4 dr. and consisting of resin (containing shellac) 10.6 per cent, magnesium-aluminium alloy (90/10) 30 per cent, strontium nitrate 59 per cent, volatile matter 0.4 per cent. The inclusion of aluminium increases the brilliance of the trace and the time of burning. The tracing composition is primed with a pellet of priming composition similar in appear-ance to the main tracer filling but containing small grains of black powder. The priming composition surrounds a smaller perforated ignition pellet of fire-grained gunpowder.

The pellets are contained in a cylindrical recess formed in the base of the tracing com-position. The compartment is closed at its base end by a zinc washer and paper disc varnished to its upper surface.

The trace burns for approximately 9.4 seconds and averages 2,680 candle power when spinning at 30,000 r.p.m.

Fig. 24

The tracer igniter is of the same design as that in the British 40 mm shell Mark I. The body has a flange at its base and is recessed and screwthreaded to receive a pellet hol-der which screws on to a copper sealing washer. The recess is prepared at the base to accommodate a percussion cap kept in position by a brass washer and, at its forward end, a hammer held off the cap by a stirrup spring. The pellet holder is recessed at the top and bottom to form a chamber for the magazine and upper part of the hammer res-pectivaly, the recess are connected by a central channel. The magazine contains a gun-powder pellet and is closed at the top by an aluminium washer with a paper disc on its under side. The central channel is filled with pressed powder and has a rough surface to prevent the part shaped to form a round headed striker and is prepared with two flash channel leading to a recess at its upper end. The stirrup spring is cylindrical with flanges formed by cutting the metal and bending it inwards to support the hammer and keep it clear of the cap until the gun is fired. The cap contains detonator composition.

Fig. 25

On acceleration, the hammer sets back, overriding the flanges of the stirrup spring and strikes ther percussion cap. The flash from the cap passes through the flash channels in the hammer and fires the gunpowder in the central channel and magazine which in turn ignite the tracer composition via its ingnition and priming pellets. The trace enables the flight of the shell to be observed. If the fuze has not already functioned the trace even-tually detonates the self-destroying detonator, which in turn detonates the bursting charge of the shell.

This fuze and gaine are described elsewhere in this pamphlet under the above heading.

The cartridge case is 12.25 inches in length and is of the normal Bofors type with a groove immediately in front of the flange and another in the base. Cases may be identi-fied by the stamping "4 cm 28" or "4 cm 28 St." on the base. The former is a brass case and the latter a steel one with a coating of brass. The case is secured to the shell by in-denting it into a cannelure near the base of the shell.

The propellant charge indicated by the black stencilling on the case is 275 g (9 oz. 11 dr.) Ngl.R.P. – (234 x 2,2/0,8) and is fitted with an igniter. The analysis of the propellant and igniter of the round examined are as follows:–

Propellant – Nitrocellulose 55.92 per cent, nitroglycerine 34.07 per cent, diphenylamine 0.76 per cent, ethylcentralite 5.15 per cent, unidentified oily substance 4.10 per cent.

Igniter – Nitrocellulose 93.25 per cent, nitroglycerine 3.31 per cent, diphenylamine 1.27 per cent, graphite 0.52 per cent, volatile matter 1.65 per cent.

The percussion primer of the round examined was the C/33 St. The primer is of the same design as the C/33 described in Pamphlet No.7, page 45, except that the body is of steel instead of brass.

In addition to the round described, two other rounds have been examined and found to be fitted with shell containing a bursting charge of 1 oz. 5 3/4 dr. of PETN/Wax (90/10) which will give an increased damage effect. The shell did not bear a nature of filling num-ber but the red stencilling on the shell body above the driving band included the abbre-viation "NP" denoting the nature of filling.

The tracer composition of these rounds did not contain aluminium; its analysis is as fol-lows:–

Strontium nitrate 50.8 per cent, magnesium metal 31.2 per cent, barium nitrate 3.2 per cent, resinous matter 14.8 per cent.

One cartridge case was of the original Bofors design and was fitted with the larger Ger-man percussion primer C/12 described in Pamphlet No. 4, page 10, and the other with the smaller German percussion primer C/13 described in Pamphlet No. 7, page 44.