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Bison concrete mobile pillbox. Over 200 were made and issued.
1280px-Fully_enclosed_Bison_2342A1.jpg

There was also a variation of the theme using asphalt and pebbles between wooden boards as armour.
 
One well aimed hit from a heavy caliber weapon and those pebbles would be like buckshot to the defenders...[/QUOTE]
It would need to be quite heavy to kill off all the crew. But it was firstly not intended to be facing artillery but light weapons of paratroopers. It's mobility was not tactical but did allow it to move itself into a place when needed without being an obstruction otherwise. Hence it was much used on airfields. Secondly (and here I draw upon my grandfather's Home Guard work) the crew were not necessarily expected to survive, as long as they delayed and drew down the enemy their job was done. Proof against 7.92mm, grenades and probably anti tank rifles. Overhead cover was not universal so mortars and dropping grenades could take early versions out. Good concrete is different to steel armour. It spalls externally as any examination of 'used' concrete pillboxes and buildings will demonstrate. FWIW one of the advantages of asphalt/pebble armour (and that was applied to lighter naval vessels too) is that part of the force of a bullet is expended in forcing the pebbles sideways deeper into the neighbouring asphalt matrix. A little like pierced steel armour where the armour sheet has holes drilled evenly across the flat plate. A bullet either enters the (too small) hole and expends it's energy laterally in trying to wedge open the hole or the plate locally bends to deflect the bullet into the hole. Thus you can get lighter armour. But I digress.
 
It would need to be quite heavy to kill off all the crew. But it was firstly not intended to be facing artillery but light weapons of paratroopers. It's mobility was not tactical but did allow it to move itself into a place when needed without being an obstruction otherwise. Hence it was much used on airfields. Secondly (and here I draw upon my grandfather's Home Guard work) the crew were not necessarily expected to survive, as long as they delayed and drew down the enemy their job was done. Proof against 7.92mm, grenades and probably anti tank rifles. Overhead cover was not universal so mortars and dropping grenades could take early versions out. Good concrete is different to steel armour. It spalls externally as any examination of 'used' concrete pillboxes and buildings will demonstrate. FWIW one of the advantages of asphalt/pebble armour (and that was applied to lighter naval vessels too) is that part of the force of a bullet is expended in forcing the pebbles sideways deeper into the neighbouring asphalt matrix. A little like pierced steel armour where the armour sheet has holes drilled evenly across the flat plate. A bullet either enters the (too small) hole and expends it's energy laterally in trying to wedge open the hole or the plate locally bends to deflect the bullet into the hole. Thus you can get lighter armour. But I digress.
Sure, that was designed to create a delaying action, but eventually heavier force would be applied to dislodge the defenders. And the pebbles, while not being entirely fatal to the occupants, could cause serious injuries when sent flying about.
A friend of my step-dad was part of an armored unit, first seeing action in North Africa with the early M4 (they had M3s on hand as well) and he had mentioned that while some Axis anti-tank guns wouldn't knock out their Sherman, the dislodged rivets would ricochet about the interior, causing serious injuries.

So in essence, the defenses have refused penetration by the enemy's offensive fire, but the crew attrition by way of injuries reduces their performance.

A RAF Liberator struck by bombs from an overhead aircraft during a raid on Monfalcone, Italy.
That is just a nightmare about to happen...was the crew able to escape safely?
 
It landed back at it's home base safely, with a large hole near the top turret. I've got a pic somewhere, showing a couple of the crew sitting in the hole in the top of the fuselage.
 
Sure, that was designed to create a delaying action, but eventually heavier force would be applied to dislodge the defenders. And the pebbles, while not being entirely fatal to the occupants, could cause serious injuries when sent flying about.
A friend of my step-dad was part of an armored unit, first seeing action in North Africa with the early M4 (they had M3s on hand as well) and he had mentioned that while some Axis anti-tank guns wouldn't knock out their Sherman, the dislodged rivets would ricochet about the interior, causing serious injuries.

So in essence, the defenses have refused penetration by the enemy's offensive fire, but the crew attrition by way of injuries reduces their performance.

At the time what was the alternative? Improvised defences like these and the Home Guard were accepted as being trading lives for time. Yes the crew would have been safer in a post 1945 Centurion tank or in trenches or thick walled pillboxes but this was doable right then and could be moved into position at short notice. To illustrate the point. My grandfather's two Vickers MMG pillboxes had no escape plan. They were in open ground. The next line was the village boundary from which rifle and LMG fire could protect the pillboxes from assaults that reached to their sides or rear but the occupants would have had to run over 200 yards of open ground under fire to escape to the next defences. War is dangerous and soldiers die. The risk was accepted, if not enjoyed. The pillboxes role was to engage the invaders over the open ground behind the beaches and dominate them for as long as possible to delay their inevitable advance (together with similar defences in the villages behind) until a proper counter attack could be made by the regular army. They lay within the preplanned defensive artillery fire plan. Similarly the entire village platoon had no plans for falling back to the next village platoon's defences. The Bison etc. were far from ideal and compared poorly with purpose built armoured vehicles or bunkers but you had to use what you could get at the time. By 1943 the Home Guard could probably have dealt with the USM all by itself but not in 1940. For the pedantic, yes I do know that most Bisons were not used by the Home Guard. It was brilliant use of otherwise unused resources in point defence. Quickly superceded but is a wonderful example of the best not being the enemy of the good. By the Grace of God none of this was put to the test.
 
That's the one. There's also a close-up shot of the damage, with at least one crew member in the pic.

Great stuff. That's one war-weary Liberator. It also clears up the lack of nose turret noticeable in the airborne shot. Thought perhaps it was a B-24D or such like, but nope, the turret's been replaced by a fairing.
 

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