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5cm M –shell fur bordkanone Rheinmetall BK-5:

pzgr40

Well-Known Member
Premium Member
Bordkanone Rheinmetall BK-5:

As WW2 progressed , it became clear that existing aircraft guns were too light to effectively combat bombers which were increasingly armoured. Weapons like the 3 cm Mk-108 gun were effective against bombers ; 3 to 5 hits on a heavy bomber were enough to bring it down, however the short range and curved flight path (low velocity) of the Mk-108 projectiles meant the enemy plane had to be approached quite close to ensure –enough- hits. The 3 cm Mk101 gun projectile was much faster and had a flat projectile trajectory, however the rate of fire was much lower (meaning less chance of multiple hits). As most bombers were heavily armed with machine gun turrets this was very risky business. Therefore it was decided to develop a heavier weapon that could fire from a safe distance, and would only need one hit to ensure a shot down enemy bomber; the BK-5.

The BK-5 gun is the result of the adaptation of the tank gun Kwk 39/1 to aeronautical use.
Theoretical the BK-5 could be used towards any aerial or ground target but his main reason to development was that the five centimeter shell promised the definitive destruction of a 4 engined bomber with a single shot.

The Kwk 39 was used in the Panzer III ausf J, L, M, and the heavy armored car "Puma", it was also projected to mount in the recce tank "Leopard" wich never saw service.
The gun is single shot, 60 calibres long, with a recoil operated hydro pneumatic recuperation system and semiautomatic breech, meaning that in the last mm of the recoil the chamber is opened and the spent shellcase is extracted. Once the new round is inserted (manually) the breech automatically closes and the gun is ready to fire.

The modifications from KwK/39 to become the BK-5 were:

A) Barrel length; the barrel was shortened about 0.5 m and provided with a single
chamber multiple perforated muzzle brake.

B) Loading; to improve the rate of fire the manual loading was deleted, and a pneumatically assisted system was added. The barrel in the recoil movement engaged a series of electric switches and mechanichal levers. These were meant to command a rammer, an extractor and the mechanism to turn the closed ammo belt.
The 2400kg recoil of the weapon was alleviated by a hydraulic damper.

The feeding was provided by a closed belt of 22 rounds, the rate of fire was 45-50 rpm. Pneumatic power came from 2 big compressed air bottles

The BK 5 was installed in the Me-410, some variants of the JU-88, JU-288 and He-177.

Characteristics:

Operation system: Pneumatically assisted long recoil.
Barrel lenght :2700 mm.
Total lenght :3790 mm.
Weight: 520 kg.
Rate of fire: 45-50 dpm
Ammunition: 50mm x 420R (KwK 39, Pak 38)

Browsing the page Luftwaffe [LuftArchiv.de - Das Archiv der Deutschen Luftwaffe] 2.WK
I have found some interesting information of the use of this weapon in the me-410A-1/U4.

Quote:
”Especially with the use of the 5cm M –shell remarkable results have been made; a group of 53 ME 410 A1/U4 planes -each equipped with a BK-5- has shot down 129 B-17 “flying Fortresses” and 4 B-24 “Liberators” in six flights between 22 Februari and 11 April 1944.
This with a loss of just nine planes on our side. On 9 and 11 April, nine B-17’s were shot down from a plm, 800 mtrs distance. This proves the great value of the BK-5.”

Description of the 5 cm M-granate (M-shell):

The projectile exists of a thin walled drawn steel body, rolled inward at the top.
It is fuzed with the AZ39 (Impact fuze 39), either the ZZ-20 (Zerlege Zunder / Self destruct fuze 20).
The projectile is filled with 335 grams of cast HTA (Hexogen, TNT, Aluminium), and has a 12 grams –cylinder shaped booster charge of NP10 (Nitropenta with 10% wax) below the detonator.
The tracer at the base of the projectile burns dimmed for the first 0,25 seconds (prevents blinding of pilot), followed by 3,3 seconds of bright tracer.

Functioning of the AZ-39 impact fuze:
The AZ-39 is an all aluminium fuze. A slider prevents the firing pin (red) from moving downward. The slider is held in place by two radial placed pins that fall in recesses in the side of the slider. These two pins are held inward by a bronze clock spring. Upon firing these two pins are thrown outward (centrifugal force), releasing the slider. A steel ball is placed in an axial hole that is placed under a 2 degrees outward angle to the top of the fuze. When fired, the negative acceleration keeps the ball in the lower position, as soon as the velocity is constant, the centrifugal force will make the ball creep upward until it falls into the hole in the slider. As the slider becomes heavier on this side it will pull the outward, releasing the firing pin. On impact the firing pin is driven into the upper firing cap (green) of the VC-70 detonator.
This detonator has a delay mechanism build in, delaying the explosion with 10 cm of projectile flight path. This ensures the projectile will explode inside the target instead of “on contact”. When the firing pin (red) hits the firing cap (green) the flame travels through the small channel into the ring formed chamber where it has to pass through another small channel to reach the lower firing cap (yellow). This will activate the detonator (white)

The shellcase used is the 50 x 420R, the same as used in the KwK/39. It is electrically ignited by means of the C/22 igniter. The shellcase is a copper clad steel shellcase.
The powder charge exists of 830 grams Digl.R.P. 310 x ø2,5 x ø1,0 (hole) powder sticks and a booster charge of 10 grams NZ.Man. Nd.P 1,5 x 1,5, placed below the main powder charge.
The powder sticks are placed in a silk bag and held together with to silk wires.
The maximal barrel pressure is 2400 Bar (At)

Vo. of the projectile : 920 mtrs/sec
Weight of complete cartridge : 3500 plm. 40 grams
Weight of projectile : 1520 plm. 20 grams
Lenhth of the complete cartridge: 606 mm
Length of the projectile: 226,5 mm


Regards , DJH
 

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Hi There,

Very interesting BK-5 info.

Would the non-tracer HE projectile have been used in this aircraft gun for air to air use or was it just the Mine shell?
Also, as far as i understand, aircraft fitted with this gun were used in the ground attack role firing the heavy AP projectiles.

I have complete HE & AP rounds but suspect the M-shells are pretty rare!

BEAR.
 
Hi Bear, as far as I know the "normal" thick walled HE shells were not used in the aircraft guns. There is no description that states that.
Concerning the Anti tank role: I only know of the 3cm and 3,7cn BK being used in the anti tank role, I have no info the 5cm Pzgr was used in the anti tank role.
Maybe one of the other members has more info on this subject.

Yes, the BK-M shell is quite rare, so when one popped up at a collectors meeting for sale I was quick to react and buy it.
Regards, DJH
 
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Hi Bear,
bottom right:
Verwendung (use) im Luft- und Erdkampf (in Air and Groundfight)
An 5cm Pzgr Patr (AP) is listed mainly for use against tanks.
Bob
 

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Here a Pzgr datasheet from June 1944.

It is just a manual, Tony or other members will have more to say about actual use.
Bob
 

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pzgr40 & Nabob. Many thanks for your replies, especially your attachments Nabob, they have answered my questions fully.

pzgr40, as doppz92 requested, any chance of posting some pics of your BK-M. And being really nosey, how much did you purchase it for?

Best Regards,
BEAR.
 
Hi pzgr40, can I push my luck and ask for pictures of your BK-M?

???:tinysmile_hmm_t2:
That's the one as described in #1.
The picture with the two shells standing up straight is the comparison between the shell as used in the PAK (the left one with the reduced bag charge) and the right one (The M-shell) for the BK.

Regards, DJH
 
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Here a Pzgr datasheet from June 1944.

It is just a manual, Tony or other members will have more to say about actual use.
Bob

I was not aware that the Bk 5cm was also used for tank hunt using trhe 5cm PzGr. I thought it was mainly meant for hunting bombers.
 
Quote:
”Especially with the use of the 5cm M –shell remarkable results have been made; a group of 53 ME 410 A1/U4 planes -each equipped with a BK-5- has shot down 129 B-17 “flying Fortresses” and 4 B-24 “Liberators” in six flights between 22 Februari and 11 April 1944.
This with a loss of just nine planes on our side. On 9 and 11 April, nine B-17’s were shot down from a plm, 800 mtrs distance. This proves the great value of the BK-5.”

I seem to recall reading that this claim was far removed from reality.

 
From the L.Dv.4000-10 of 1944.
 

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???:tinysmile_hmm_t2:
That's the one as described in #1.
The picture with the two shells standing up straight is the comparison between the shell as used in the PAK (the left one with the reduced bag charge) and the right one (The M-shell) for the BK.

Regards, DJH


:tinysmile_fatgrin_t yes of course, sometimes, I am not really sharp... I meant the other side, with stencils if there's any.
 
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There are no stencils present, the projectile was lightly polyestered and repainted. Even I would probably refrain from cutting up a mint 5cm M-shell with Original paint marks and wait untl I could get a lesser one ,either cut it but leave the paint stenceling untouched.

@ Rigby....WOW... that's new for me! I was not aware the "normal" 5cm SprGr and Pzgr were used in the 5cm BK, only with a different powder charge for 5cm SprGr (compared to the Pak 38) in the shellcase. Just learned something new!!
 
I was not aware the "normal" 5cm SprGr and Pzgr were used in the 5cm BK, only with a different powder charge for 5cm SprGr (compared to the Pak 38) in the shellcase. Just learned something new!!

The standard HE loading fired by tanks and AT guns used a reduced charge compared with the AP loadings. That was to allow relatively thin-walled, high-capacity HE shells to be used.

British and US tank/AT ammo initially fired HE shells at the same velocity as AP, to simplify aiming. However, that meant that the shells had to be thick-walled to withstand the pressure, so carried less HE. (The US tankers complained when the 76mm tank gun was introduced, because the HE shell contained much less HE than the lower-velocity 75 mm HE). The British downloaded 17 pdr HE later, so they could use higher-capacity shells.
 
I always learned the HE shells had a reduced charge to reduce barrel wear.
The story does no add up, as the normal SprGr. for PAK gun usage would use a reduced charge because if the shell wall thickness, and the same shell for aircraft usage would use the full charge as depicted above, let alone the even more thin walled M shell that has an even thinner base and wall thickness, using the full charge.
I can only think that the sights of a Pak gun can be adjusted to PzGr and SprGr usage quite easily on land, something quite impractical in an airplane, so the trajectorys were equalized to simplify aiming.
 
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I think that the issue is related to chamber pressure, which is what mainly determined the stress on the shell. A lightweight shell like the M-Geschoss was easily accelerated down the barrel, so generated a lower chamber pressure than its velocity might suggest; the shell could therefore be made of thin steel. An HE shell of standard weight would accelerate more slowly and therefore (other things being equal) generated a higher pressure, so the shell needed to be more strongly made to resist that pressure.
 
A fascinating thread. Was the same case used with the BK with same markings or were the markings different?
 
Data the 5cm Sprgr for BK5 has 900+30g of powder and V0=870m/s
The Pak Sprgr has 236+30g and V0=550 m/s.
IMHO it is the muzzle velocity that is the important factor here and therefore the external ballistics determines the internal ballistics.
I think there were no mixed loads of 5cm HE and AP projectiles to mess with the aiming sights. Maybe not even Sprgr and M-Gesch. mixes
DJ is certainly correct that in ground weapons, the load of HE rounds is much smaller and this significantly reduces wear. As kinetic energy of HE projectile is not too important (the explosive charge makes most of the work) only the practical amount of powder is used to send the HE round downrange. (Now we talk about tank and anti tank guns with limited elevation nor AA guns or high power canons).
The mechanical stress of firing is certainly relevant and is computed and tried when a round is designed. With 50mm projectile I would be more afraid for the driving band than the projectile base.
As projectile weight goes it is not as important feature in internal ballistics as many others related to maximum chamber pressure.
So my conclusion is that the large powder charge is needed to give the round flatter trajectory and shorter time of flight to the target. The practical barrel life of the BK/KwK was probably higher than the engine life anyway.
Source of pictures here: https://www.deutscheluftwaffe.com/a...esserschmitt/Me 410/Me 410 A-1 U4 Wa Bk 5.pdf
Bob
 

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I think that the issue is related to chamber pressure, which is what mainly determined the stress on the shell. A lightweight shell like the M-Geschoss was easily accelerated down the barrel, so generated a lower chamber pressure than its velocity might suggest; the shell could therefore be made of thin steel. An HE shell of standard weight would accelerate more slowly and therefore (other things being equal) generated a higher pressure, so the shell needed to be more strongly made to resist that pressure.

It is related to chamber pressure, but not exactly due to the reason you describe it seems in this case. Yes, in general, if the same charge and charge type was used and the projectiles had the same dimensions, then the heavier projectile would result in a higher chamber pressure than the lighter projectile.

The max pressure the M-Geschoß and the standard shell will be able to withstand would not be down to just their thickness, but also what they are made of and how they are made (treated). The two projectile types would have been designed so that both would be able to withstand roughly the same base pressure. Other factors also come into play, such as the effect of acceleration on the side walls of the projectile. I have seen first hand - well not exactly first hand as I was using a high-speed (down to 100s of nanoseconds) digital camera to take the images - of side-wall collapse on cannon-calibre case-telescoped ammunition-fired projectiles due to high acceleration and thin, not strong enough sidewalls. On firing the sidewalls on some of these projectiles failed, resulting in the walls collapsing inwards. Thankfully they were inert-filled, so there were no in-bore prematures due to the HE-fill being pinched by the walls.

Anyway, back the M-Geschoß/standard type shell problem. In this case it's the length of the M-Geschoß that makes the difference. Going by the 1944 publication, 'LDv. 4000-10 - Munitionsvorschrift für Fliegerbordwaffen', all MG 151/20 projectiles use either Nz.R.P. (1.3×1.45/0.2) tubular (the 'R.P.' bit) or Nz.Bl.P. (2.0×2.0×0.7) rectangular (the 'Bl.P.' bit) single-base (the 'Nz' bit) propellant. I plugged the data in for each type of projectile into the internal ballistics program I use when developing hand-loads (QuickLOAD). I found a suitable powder (Reload Swiss RS 70) and gave each projectile the same powder charge. The results are below as screen captures.

MG151-20 2-cm M-Geschosspatrone type projectile.png

The first screen capture shows the 92 gram (1,420 grains), 84 mm long (3.307") M-Geschoßpatrone type projectile, its case intrusion 19.5 mm. The max pressure generated was 38,693 PSI (266.8 MPa), its MV 2,470 ft/s (753 m/s).

MG151-20 Brandgranatpatrone type projectile.png

The second screen capture shows the 115 gram (1,775 grains), 80 mm long (3.150") Brandgranatpatrone type projectile, its case intrusion 15.5 mm. The max pressure generated was 39,185 PSI (270.2 MPa), its MV 2,307 ft/s (703 m/s).

So the same charge was used, but one projectile weighed 92 grams, and the other 115 grams, the pressure difference between the two being only 492 PSI (3.39 MPa), which is frankly nothing really.

So in the case of the MG 151/20 and those two projectiles, its the length to which the M-Geschoß intrudes into the case, which decreases the chamber (available case) volume, that results in a similar pressure, even though it weighs far less.
 
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So in the case of the MG 151/20 and those two projectiles, its the length to which the M-Geschoß intrudes into the case, which decreases the chamber (available case) volume, that results in a similar pressure, even though it weighs far less.

Thanks for this, you are right of course that the M-Geschoss aircraft shells reduce the space allowed for propellant.

Another factor affecting gun performance is recoil: the MG-FF was the first Luftwaffe cannon to use M-Geschoss shells (from mid-1940) and it was found that the propellant + light shell combination did not generate enough recoil to operate the automatic gun mechanism, even when stuffing the case with as much propellant as possible. It was necessary to reduce the weight of the bolt and the strength of the recoil spring (creating the MG-FFM) which then meant that the gun could not use full-power (not M-geschoss) ammunition; when tracer shells were wanted, the heavy standard HE-T shells were used but reduced in weight by using light-alloy rather than brass fuzes, and downloading the propellant so that the MV was only 585 m/s.
 
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