US 20mm cartridges - T7?

I am currently trying to pin down the 20mm automatic cannon cartridges being tested by the US in 1945-50, specifically those for the ARF T33 and T74 guns, but am becoming rather confused by Chinn (not for the first time!).

I understand that the T74 cannon was just the US designation for the Mauser MG 213C revolver gun, of which the US had acquired an example in 20mm calibre - therefore chambered for the 20 x 135 Mauser cartridge.

I also understand that the T33, T33E1 and E2 were chambered for that giant 20 x 158RB cartridge, which had the case designation T5. This (I think) fired a 2,000 grain projectile at 3,500-3,775 fps.

But Chinn then says that the T33E3 was chambered in a cartridge with a smaller T7 case which fired a 1,600 grain T61E1 projectile, also at 3,775 fps, and which had a 900 grain propellant load. This round was supposedly designed originally for the T74, but I can’t imagine that they were intending the adopt the Mauser 20 x 135 case…

Can anyone please tell me anything about the T7 cartridge case, especially its dimensions?

Damn. No replies. I am also interested in this subject.

More info has emerged since:

After the war, an example of the MG213/20 arrived in the USA where it was carefully analysed under the designation T74, and various modifications made. It appears that the 20 x 135 ammunition, designated T7, was retained for these analyses, with brass rather than steel cases manufactured, and it was probably also used in the experimental ARF T33E3 gun. The loading for this ammunition reportedly used a conventional 104 g projectile (T61E1 – a shortened version of the 20 x 110 Hispano M99 TP projectile) at an MV of 1,150 m/s. Chamber pressure was 338 MPa.

That’s really awesome performance. Can I have a GAU-22 rechambered for it? It’s, uh, for my car…

Tony - I found a T7 case although I had never heard of it until I found this thread. Let me know if you need photos or any info


Mike, please show it to us. I am sure this is what Tony was asking for (at least partially).

Thanks Mike, a good catch. I have a complete German 20 x 135 round for the MG 213/20.

Does anyone have a photo of a complete T7 cartridge? I see it used a modified M99 projectile, would like to see the modified one!


Thanks for the photos.

Can you please provide the exact case length? In millimeters if possible.

With the one photo you posted with the tape measure next to the T7 case it is difficult to see the exact case length.



This one in the middle is a modified M99 projectile, possibly the one mentioned?
M90 series(M99A1), M99 (?) and M50 series(M56) projectiles


my post from BOCN, note’
This one i got as a ‘stuffer’,in a US Hispano(M21A1) case, the only markings i can find on the damaged driving band are; -7-45-20MM M .
This is shorter and different profile to the standard WW2 vintage M99 practice projectile,being closer to the M50 series projectile of the 1950s-.
My understanding is these projectiles were used with the development of the .60/20mm (20x103mm/M50)T61E1 case type from the original .60cal round in the late 1940s.

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20x135R32 with a slight bevel around the edge of the rim

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