Muzzle velocity of 20mm ShVAK shells

[KillaKiwi]

The exact muzzle velocity seems to be a bit of a mystery but I’m gonna share the informaton I’ve come across.

In general it can be said that the muzzle velocity is greatly influenced by the barrel lenght.

The default barrel lenght is 1250mm while the TnSh-20 (Tank version) and motor cannon have a barrel lenght of 1540mm, with the motor gun reaching 1700mm with an unrifled barrel extension at the end.

20mm ShVAK MP

Original the ShVAK fired the OF (HEF) and OFZ (HEFI) as well as a Tracer round which all weighed 91g.

91g OFZ with 0.85g Tetryl and 3.9g Flashpowder

Muzzle velocity is generally shown with being 800m/s but 815m/s figures also exist.
According to “Soviet Cannon - A comprehensive Study of Soviet Guns and Ammunition in Calibers 12.7mm to 57mm - 2009” a Hungarian manual gives a velocity of 815m/s for all shells for the 1540mm long barrel.

I also came across some Russian document showing the velocity, flight time and impact points from the gun sight, for the 7.62mm ShKAS, 12.7mm Berezin and 20mm ShVAK.

Soviet aircraft guns ballistics

With the Berezin shown having a velocity of 860m/s and the ShVAK having a velocity of 815m/s.
The Berezin is refered to as BS, meaning an older version from ~1939.
So we can conclude that that the velocity of the ShVAK would refer to the pre-war shells that weighed 91g. However it’s not clear what barrel lenght the ShVAK would have.
In 1935 the ShVAK was proposed to be used as AA gun on the ground, so it’s likely that a longer barreled version already existed at this point, which makes it likey that the 815m/s figure referes to 91g shells fired from long barrels.

However the manual for the T-60s TshN-20 cannon shows that projectiles are fired with a muzzle velocity of 800m/s and a Soviet document testing the armor piercing capabilities of the AP round, shows the velocity to be 796m/s using regular propellant powder.

Autocannon Penetration

The TshN-20 uses the long 1540mm barrel, yet the shell only reach ~800m/s firing ~96g OZ and BZ shells.

96g OZ shell with 5.6g A-IX-2

96.6g BZ shell with hardened steel penetrator and 2.8g flash powder

The ballistic table gives similiar flight times to the table for 815m/s shells. Only after 600m the TshN-20 shells are falling behind the table with 815m/s veloctiy, suggesting some difference in ballistics.

20mm TshN Ballistics

(*)“Soviet Cannon - A comprehensive Study of Soviet Guns and Ammunition in Calibers 12.7mm to 57mm - 2009” also writes that Germany tested captured shells, which resulted the following velocities for a 1250mm barrel:

• 91g OF → 790m/s
• 96g OZ → 768m/s
• 96.6g BZ → 751m/s

A German document dated 1943 about enemy aircraft ammunition shows both shells having a velocity of 750m/s. However the information in that document about foreign ammunition can not be considered 100% reliable.

Spoiler

We can make the following assumption:
With the 1540mm barrel the old 91g shells reach a velocity of 815m/s, with the 1250mm barrel around 790m/s.
With the 1540mm barrel the ~96g OZ and BZ shells reach a velocity of 790-800m/s but only around 750-770m/s with the 1250mm barrel.

The same source(*) states that the propellant powder charge for all shells is 18g of VT powder.

For the later shells the gas pressure in the chamber reaches 3000at.

20mm OZ, OZT, BZ shells with 18g propellant, chamber pressure 3000at

In comparison, the German 20mm Mineshell fired from the MG 151/20 with a 1104mm long barrel reaches the following velocities, according to “Handbuch der Flugzeug Bordwaffenmunition 1936-1945”:

92g shell -> 780m/s with 19g propellant charge, 3100at pressure, electrically fired

92g shell -> 775m/s

92g shell -> 790m/s, electrically fired

95g shell -> 755m/s with 19g propellant charge, 2850at pressure

95g shell -> 755m/s

95g shell -> 770m/s, electrically fired

106g 20mm Incendiary 44, 745m/s with 19.5g propellant charge, 3150at max. pressure, electrically fired

We can see that the 95g Mineshell comes very close to the weight of the Soviet OZ, OZT and BZ projectiles. Reaching 755m/s with a slightly shorter barrel from the striker fired MG 151/20.

From what I know, the 20mm FlaK 38 fired a 148g AP round with 830m/s, using a L/65 (1300mm) barrel. With the shorter L/55 barrel (1100mm) for armored fighting vehicles, this dropped down to 800m/s. A similiar velocity drop then what the ShVAK with 1250mm barrel probably experienced.

[KillaKiwi]

Spoiler

Some more data.

Velocity for the B-20 cannon is shown with 750-790m/s, which obviously would only fire the post 1941 ammunition.
Velocity for the ShVAK gun firing 91g shells is 815m/s. The propellant charge is 19.2g compared to the the later improved shells (OZ and BZ), which were longer and heavier, resulting in only 18g of powder with reduced velocity.
However it’s shown that the 815m/s velocity is achived from the shorter barrel.

Considering that the long barrel version was only employed later as motor cannon, maybe after previously considering it as AA gun with long barrel, this seems to make sense that early ShVAK cannons would use the 1250mm barrel, firing 91g shells at 815m/s.

Of course this still is conflicting with the German test data from 1941 were the OF shell only reached 790m/s and the OZ just 768m/s.

“Soviet Cannon - A comprehensive Study of Soviet Guns and Ammunition in Calibers 12.7mm to 57mm - 2009” states that the B-20 also had an electrically fired version with a ~1100mm barrel.
This could explain the wide margin in velocity being shown with 750-790m/s, or it simply refered the optimal and lowest velocity depending on the wear of the barrel.

[Ghostmaxi]

But i wonder now, how should such be implemented in game?
C&P the different guns, change velocity and add the Barrle length in the name behind it?
Would be nice to see such implemented.

[KillaKiwi]

@Ghostmaxi

C&P the different guns, change velocity and add the Barrle length in the name behind it?
Would be nice to see such implemented.

I think kinda the same with the Berezins. The default model is the UB but in aircraft it’s either UBK (wing), UBS (synchronised) or UBT (turret).

So we would need to differentiated between:

• SP-20 (synchronised or wing mounted) - 1250mm barrel
• MP-20 (motor cannon) - 1540mm barrel
• TP-20 (turret) - 1250mm barrel

[KillaKiwi]

@KillaKiwi

Of course this still is conflicting with the German test data from 1941 were the OF shell only reached 790m/s and the OZ just 768m/s.

If test were carried in out in 1941 with a 1250mm version, this could imply that the gun was taken from an IL-2 ground attack aircraft, since in 1941 I can’t recall any other aircraft that would use a 1250mm barrel version of the ShVAK that the Germans could have encountered.
The barrel might have been somewhat worn out and the AP shell might have been heavier then in later productions.

Which just makes one thing clear, that the 91g shell would be 20-25m/s faster out of the short barrel compared the 96g shell.

So out of a new barrel the velocity would probably be:

• 815m/s for 91g OF, OFZ and Tracer
• 790-800m/s for the 96g OZ, OZT and BZ shells weighing ~96g, with potentially less propellant.

815m/s could potentially be achived with heavier shells out of the long barrel.
This conflicts with the firing table for the TshN-20 which says that the shells are fired with 800m/s out of a 1540mm barrel and velocity data for the B-20 that shows velocity of 750-790m/s.

From a logical point of view it should be:

• 815m/s for 91g shells out of a 1540mm barrel
• 790m/s for 91g shells out of a 1250mm barrel
• 800m/s for 96g shells out of a 1540mm barrel (confirmed in TshN-20 testing with 796m/s)
• 770m/s for 96g shells out of a 1250mm barrel