# .303 versus U-boat WW2



## WATU (Aug 19, 2020)

Hi. Debate underway on whether an aircraft armed with .303 machine guns would penetrate the saddle tanks, conning tower and/or the pressure hull. I see the gun as for clearing crew off the hull rather than affecting the U-boat structurally. Anyone aware of any evidence for bullets penetrating a U-boat. And if so what degree of damage could be caused? Could fuel tanks be self-sealing?


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## Peter Gunn (Aug 19, 2020)

Interesting question, if you're talking something like a Hurricane with 8 or 12 and he gets the U-Boat in their convergence zone, it might do enough for them to have to effect some relatively minor repairs at sea. If you're talking about a single mount on something like a Sunderland I doubt it would do more than scratch the paint.

Of course I could be way off base, that's just my $.02.


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## WATU (Aug 19, 2020)

I was thinking of a Sunderland really so could be 1-4 guns but I assume with no convergence. Would a sustained burst weaken the plates? Given the aircraft movement and an unstable U-boat I cannot see that the impacts could be concentrated in a very small area. More of a moving sweep better for suppressing the deck crew.


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## MIflyer (Aug 19, 2020)

I have an Enfield No. 4 Mk 1. Wonderful, beautiful very accurate rifle and better balanced than my M1 Garand. But the only time I would shoot at a U-boat with it is if I was inside the boat. 

But note that on the Mosquito FB.XVIII they equipped it with a 57MM Mollins 6 pounder AT gun for antishipping, but left the four .303 machine guns. I assume the .303 were for dealing with the gun crews on the boat's deck.


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## pbehn (Aug 19, 2020)

MIflyer said:


> I have an Enfield No. 4 Mk 1. Wonderful, beautiful very accurate rifle and better balanced than my M1 Garand. But the only time I would shoot at a U-boat with it is if I was inside the boat.
> 
> But note that on the Mosquito FB.XVIII they equipped it with a 57MM Mollins 6 pounder AT gun for antishipping, but left the four .303 machine guns. I assume the .303 were for dealing with the gun crews on the boat's deck.


I think they retained the .303 for sighting.


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## stona (Aug 19, 2020)

It seems to me that unless you know the thickness and composition of a U-boat's pressure hull you are guessing.

British submarines had pressure hulls of varying thicknesses, from 0.5" to 0.865" depending on type. I don't know about U-boats, but when the British examined U-570 the report stated

_"A clean hull and welding good. Why such a thick pressure hull in association with relatively flimsy frames...."_

which would imply both that the principles behind submarine construction were different and that the German pressure hulls were constructed from material thicker than that used by the British on their submarines.

There were various Marks of .303 AP ammunition used and most were required to penetrate 8mm to 10mm, usually at 100 yards. Even without knowing the composition of the material used in the pressure hulls of the U-boats, it seems that their pressure hulls were even thicker than that of contemporary British submarines which would surely make them impenetrable by any .303 ammunition used by the British in WW2.

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## MIflyer (Aug 19, 2020)

pbehn said:


> I think they retained the .303 for sighting.



No way the trajectory would be anywhere near close.


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## pbehn (Aug 19, 2020)

Maybe the effect would be psychological, would you take it down to the depths of a WW2 movie after being hit by gun fire, while inside you don't know the calibre of gun that hit you.

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## pbehn (Aug 19, 2020)

MIflyer said:


> No way the trajectory would be anywhere near close.


Probably not but better than nothing from Wiki Another fighter-bomber variant was the *Mosquito FB Mk XVIII* (sometimes known as the _Tsetse_) of which one was converted from a FB Mk VI to serve as prototype and 17 were purpose-built. The Mk XVIII was armed with a Molins "6-pounder Class M" cannon: this was a modified QF 6-pounder (57 mm) anti-tank gun fitted with an auto-loader to allow both semi- or fully automatic fire.[nb 23] 25 rounds were carried, with the entire installation weighing 1,580 lb (720 kg).[142] In addition, 900 lb (410 kg) of armour was added within the engine cowlings, around the nose and under the cockpit floor to protect the engines and crew from heavily armed U-boats, the intended primary target of the Mk XVIII.[179] Two or four .303 (7.7 mm) Browning machine guns were retained in the nose and were used to "sight" the main weapon onto the target.[142]

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## Warspite (Aug 20, 2020)

When used against a U-boat, the only thing the .303 machine guns were good for was keeping the submarine crews heads down and away from the AA guns.


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## Greyman (Aug 20, 2020)

British testing revealed that 20mm Hispano AP and 40mm Vickers AP was generally ineffective against a U-Boat pressure hull. The main problem during most attacks was the striking angle.

45 degree diving attacks could penetrate under 400 yards provided the strikes were 'dry' -- but even just 2 feet of water was enough to destabilize a 40mm round and render it ineffective.

_Numerous trials have been carried out with 20 m.m. against submarine targets. As a result of these Air Staff, Air Ministry, have decided that 20 m.m. is ineffective as a primary weapon for destroying submarines. 50 calibre even less effective. Projects for operational installation of 20 m.m. for this purpose have now been abandoned.
 - Ministry of Aircraft Production_

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