Hispano Mk. II./V & US M1, AN-M2/3 20mm Ammuniton

[the_emperor]

I dug through some old hard drives and found some old  compiled pdfs for the allied 20mm aircraft ammunition which was introduced in summer ´44 to have all rounds to have simillar ballistics (TM9-1901 29th June 1944).

The rounds are the M-95 AP-T, the M-96 Incendiary and (I guess for us the most interesting round)  the M-97 HE-I round.

All of aprox 0.29 pound (131.55g) weight and 2,800fps muzzle velocity and 1.66 seconds of flight time to travel 1,000 yards.

The HE-I M97 shell carries 0.017lbs/7,71g Tetryl/Incendiary mixture (0.012lbs/5,44g Tetryl and 0.005lbs/2,27g Incendiary) and has a super-quick (S.Q.) fuze with  instantaneous action upon impact on light material (e.g. aircraft skin). No indication on any delay mechanism or charge.

The "old" HE-I Mk I weighs in at .29lbs with a filling of .025lbs (aprox the same weight of tetryl of 0.012lbs and 0.013lbs of incendiary)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Production of the old & new 20mm Aircraft ammunition (Official Munitions Production of the United States (by Months Juli 1940 - August 1945) CivilianProductionAdministration 1947):

 

Edited December 6, 2022 by the_emperor

[the_emperor]

Edit:

 

in the 1957 TM 9-1901-1, Ammunition for Aircraft Guns I found the specific HE/Incedniary Mixture for the M97 round:

0.012lbs Tetryl and 0.005lbs Incendiary.

This manual also gives us an update on the muzzle velocity if 2,730 feet/s (more accurate instruments to measure in ´57?)

Edit: the 2,730feet/s are for the M3 20mm autocannon, which has the same barrel length as the Hispano V if I am not mistaken.

Bu the manual still states approx. 1.66s flight time for 1,000 yards.

 

 

 

 

 

Edited February 9, 2022 by the_emperor

[the_emperor]

From June 1945 from the the Navy manual on fuzes and projectiles

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Edited February 8, 2022 by the_emperor

[the_emperor]

I was seaching whether the No. 253 Mk.III or M75 fuze do offer any delay action, but did not found any. But I tink one can convey something in that regard from the successor fuze the M505A3 in a quest to get some delay for an improved M505A3 fuze (longer travel and/or heavier fire pin) for the high velocity (3200-3500feet/s) 25mm rounds.

The M505A3 fuze is described in the same terminolgy as the M75 fuze and offers negligible delay. Tested at a 0° obliquity (90° angle) at 0.040inch aluminium with 2500feet/s. The testing showed that shallower angles and thicker targets and redcued velocity even reduce the delay...so one may concluded that the M75/No.253 fuze offer no delay.

 

 

 

(modified M505A3 fuze with delay function, final report 1974, air force armament laboratory)

 

Edited February 14, 2022 by the_emperor

[the_emperor]

X-ray of a M97 (M75 fuze) HE-I fired from a AN-M2  with a striking velocity of2850 feet/s at a mild steel plate (WD 1010 hot rolled annealed and pickled, or cold rolled, dead soft temper)(HIGH SPEED X-RAY PICTURES OF THE FUNCTIONING OF 20MM HEI SHELL M97 (T23) WITH FUZE P.D. T71E4 ; AD0147191; 1945-09-30)

:

1) at a 1/16 plate the shell frequently did not function until the round was completely through the plate. Two out of twenty rounds did not work at all.

The shells that that did work ruptured when more than halfway through the plate (~44 microseconds after impact) and created a ~ 2 1/8inch hole.

 

With the PD Fuze 253 MkIII the all fuzes functioned before the shell was half way through the plate and created an approx. 4 inch hole.

 

 

a) at a 0.083inch mild steel plate angeled 20° to the line of fire.The Shell ruptured before passing halfway through the plate and created a  hole of 4.5inches.

 

 

 

b) fired at a 1/8 inch plate. The Shell ruptured before the bourrelet passed through the plate (~45microseconds after impact ) and created a 6-7inch diameter hole. The PD 253 MkIII fuze functioned in the same manner.

 

 

 

c)  fired at a 1/4inch plate. The Shell ruptured before the bourrelet passed through the plate (~40 microseconds after impact) and created a hole of 5 inches.

 

at 2500f/s and 40-45microseconds time to rupture the round travels ~3-3.5cm after impact (deceleration of the impact not taken into accout; the round with M75 fuze is ~83mm long) or ~4cm at 2850 f/s

The average radial velocity imparted to the fragments by the force of the explosion was ~2800 feet/s

Here are some more details about the fragmentation of the M97 round (and its a bit contradicting):

(Handbook of balistic and engineering data for ammunition, Volume I&II, 1950)

Edited February 14, 2022 by the_emperor

[the_emperor]

20mm M 75 & M95 AP-T armour piercing perfomance (Terminal Balistic Data Bombs, Artilerry & Mortar Fire and Rocket , Volume III)

 

 

 

Edited February 14, 2022 by the_emperor

[the_emperor]

Unfortunately a major shell of the british 20mm Hispano system is missing in the game. Namely the 20mm SAPI (sources unknown):

 

https://youtu.be/1i2iAti23I0?t=464

 

 

 

 https://www.youtube.com/watch?v=GmZo6SRsrbk&t=198s&ab_channel=BritishPathé

 

Edited October 17, 2022 by the_emperor

[the_emperor]

Some drawings of early AP (no tracer), HE-I and Ball-Tracer Shells for the 20mm Hispano, also of the early No.253 MkI Fuze (notable the brass disc in the nose and the undrilled brass disc before the detonator. In the Mk.II the lower brass disc got a drilled hole to increased sensitivity of the fuze, in the Mk.III the uper brass disc was ommited.

(though the filling weight is not given, the HE-I shell filling/increments comes close to the above mentioned SAP-I: two Incendiary filling of each ~3grams with the rest 5-6grams being HE filling for a total of 11gram/0.025lbs filling. Seems the HE-Filling did not change during the Evolution to the M97 shell but incendiary was reduced)

(source unknown)

 

Edited February 3, 2023 by the_emperor

[the_emperor]

Some more types of shells for the Hispano System. Shell if the Crash type with base fuze. Scource unknowd, and I dont know how widely and when it was in use.

[the_emperor]

Though the US/British Hispano System did not employ a pure HE-Round but "only" HE-I with ~6g of Tetryl HE-payload (~1.25 TNT effectivness), the french HS404 System did use a pure HE-Shell with with a max payload of 10g pentolite (~1.33 TNT effectivness)  and a hot powder charge of 35g for 880m/s muzzle velocity. Additionally the fuze features a auto destruct function:

 

SERVICE DES COURS

RENSEIGNEMENTS SUR LES 
MUNITIONS DE PETITS CALIBRES (Tirage 46; Avril 40)

Edited February 3, 2023 by the_emperor

[the_emperor]

@VO101Kurfurst provided a german intelligence document which  specifies the amount of HE filling for the „old“ HE-I round 

with 5.6grams matching the assumptions beforehand and confirms that the HE-filling remained the same during the evolution to the M97 shell (0.012lbs~5.44grams)

Edited February 12, 2023 by the_emperor

[MDNW]

Hello Everyone,

 

New subscriber. I'm hoping someone can help identify this fuze in more detail. I've been finding multiple .50 caliber rounds and 20mm cannons on A beach in North West England called Formby Point. I've since figured out that these are from a Supermarine Spitfire of which there were a few in the area during and after WW2. The beach had multiple ship wrecks which the RAF used for target practice. This would explain all the ammunition's I'm finding.  What id like to know is, What type of firing mechanism is this head. Many Thanks The markings read MK.xxvi MOD B AC LO 446

 

 

Edited March 4, 2024 by MDNW

[the_emperor]

[MDzmitry]

Hello everyone, regarding the composition of HEI shells it should be noted that when considering TNT effectiveness (as in the quote below)...

03.02.2023 в 13:50, the_emperor сказал:

~6g of Tetryl HE-payload (~1.25 TNT effectivness), the french HS404 System did use a pure HE-Shell with with a max payload of 10g pentolite (~1.33 TNT effectivness)

...it should be noted that in practice the total mass of both the HE and the Incendiary filling should be accounted for, as well as the specific combination of them in %.
Found a research on this point, sending the web archive link, as well the Google Drive link to the file which I saved just in case:
https://archive.org/details/DTIC_ADA492532/mode/2up
https://drive.google.com/file/d/1PQb-IXqbPHvWcyetuR9RWixXaRqY219H/view?usp=sharing

 

If we're to believe the comparison of fillings C (British HEI: C.E./S.R.379) and F (R.D.X./T.N.T.), the former was 1,38 times more efficient.

Edited March 27 by MDzmitry

[the_emperor]

thx @MDzmitry thats a cool read👌

[Wingman_09]

On 3/27/2025 at 8:50 PM, MDzmitry said:

Hello everyone, regarding the composition of HEI shells it should be noted that when considering TNT effectiveness (as in the quote below)...

...it should be noted that in practice the total mass of both the HE and the Incendiary filling should be accounted for, as well as the specific combination of them in %.
Found a research on this point, sending the web archive link, as well the Google Drive link to the file which I saved just in case:
https://archive.org/details/DTIC_ADA492532/mode/2up
https://drive.google.com/file/d/1PQb-IXqbPHvWcyetuR9RWixXaRqY219H/view?usp=sharing

 

If we're to believe the comparison of fillings C (British HEI: C.E./S.R.379) and F (R.D.X./T.N.T.), the former was 1,38 times more efficient.

 

Now that's a smoking gun if I've ever seen one.

 

Though probably won't convince the sort of people who:

1. Multiply the amount of explosives by RE factor from that table on wikipedia (despite the very article it's in describing in detail why it is misleading if you don't know what you're doing)

2. Obviously conclude that the Germans™, as with their every other engineering endeavour, were geniuses for using pure PETN (even though they switched it for HA41 later).

[LukeFF]

11 minutes ago, Wingman_09 said:

Now that's a smoking gun if I've ever seen one.

 

Though probably won't convince the sort of people who:

1. Multiply the amount of explosives by RE factor from that table on wikipedia (despite the very article it's in describing in detail why it is misleading if you don't know what you're doing)

2. Obviously conclude that the Germans™, as with their every other engineering endeavour, were geniuses for using pure PETN (even though they switched it for HA41 later).

 

Let's watch the tone, please. 

[MDzmitry]

8 часов назад, Wingman_09 сказал:

Now that's a smoking gun if I've ever seen one.

Glad to see you found the info interesting, but there's no need to "throw hands" with people who didn't know about it before.

 

It's only natural to make assumptions, and it's pure luck to have this document, otherwise I myself would've kept believing the HE concept.