3 inch OSB Gun

The 3-inch OSB gun (Smith gun) was another sub-artillery weapon issued to the Home Guard for the defence of towns and villages. It was a mobile weapon which could fire either anti-tank or anti-personnel shells:

Max Most
Range (yds) Effective Range (yds)
8lb H.E anti personnel 650 150
6lb H.E anti-tank 200 100

Right: Trajectory Chart for 3"
S.B Gun Mk I.
Projectile - Bomb 3" H.E. Mk I
Fuse No 245.
Weight - 8 1/2 lbs.
Charge - 310 grains Cordite
WM 017.

Compiled by Ixworth Home

The anti-tank round could pierce 80mm armour plating at 50 yards range and could penetrate ferro-concrete and a brick wall 9 inches thick.

The gun was towed on two 4ft diameter wheels. A trailer carried spare rounds.The gun and trailer could be towed by one 10 hp car or each by a motor cycle. One of the wheels also served as the base from which the gun was fired from. The gun weighed 5 cwt. 1 qr. 16 lbs and the trailer 3 cwt. 33 lbs. The relative light weight combined with the wheel size permitted its crew to maneuver the gun easily by hand.

Above: 3 inch OSB (Smith Gun)

The gun could be fired from any roughly flat surface. If the gun was weighted with sandbags it would jump back 2 inches after each shot; if unweighted about 6 inches. This would mean it would be necessary to re-lay the gun after each shot. The gun had a low trajectory meaning it would need a clear view to fire from, which made concealment difficult. It was recommended to open the breach and look through the barrel to ensure crest clearance. This would have to be done after every change of range as a change in range would also change trajectory. When selecting a pre-prepared position, a reconnaissance stick could also be used to check crest clearance.


Above: 3" OSB (Smith Gun) in action.

Tactical handling:

In defence it could be used to cover dead ground – due to difficulties in concealment it was recommended to provide alternative positions.

It could be held in reserve by the garrison commander to reinforce any threatened point – its mobility meaning it could be quickly deployed.

In counter attack it could support infantry especially with fire from a flank.

Its mobility also meant it was suitable for standing patrols (i.e. a patrol sent to cover / watch ground from which the enemy was likely to approach from but was not expected to hold this position to the last).

The Home Guard in Suffolk was certainly issued with this weapon as, for example, shown by a surviving defence scheme for Wickham Market.


Home Guard Instruction No.51 – Part IV The Organization of Home Guard Defence, GHQ Home Forces, 1943

smith trajectory

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