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20 X 158RB T5 round


Well-Known Member
Premium Member
I finally got this a couple of days ago after many years of looking for one!!
Its an experimental 20 X 158 T5 round.
I can't find out much information on it other than what Tony Williams mentions in his book "Flying guns the modern era":
"Another US post-war experimental 20mm gun project was the T33, which was originally worked on by Oldsmobile but taken over in 1946 by the Armor Research Foundation (ARF). It was a gas-unlocked blowback obviously ifluenced by the Hispano, bur using a bolt locking mechanism mased on the Rheinmetall-Borsig Mk 103. It was initially designed around the big T5 20 X 158RB cartridge, although in 1948 a decision was taken to change to the new 20 X 102".

Does anyone have any further information on the weapon, performance of it etc?

The headstamp is quite hard to make out - what I can see is as follows:
LOT F.A..E 1211945
20MM T5

There are no stampings on the primer.

Stencling on the projectile is as follows:
20MM., M99

Stamping on the driving band is as follows:
20MM M99

Rim diameter is 30.4mm
Primer diameter is 13.1mm

All INERT etc, big hole in the back/primer oiled etc.




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That's a nice one to get, Dave!

The only source of info I know on the gun and ammo is Chinn's 'The Machine Gun'. It cost me a huge sum to buy my five-volume set many years ago, but you can now get scans on CD ROM very cheaply - worth looking for.
Thanks Tony, I completely forgot about Chinn, I've got the CD version will dig it out and do some research.
Slight correction to the title on this thread.
It should read 20 x 157/RR
The M99 projectile is the common load found, but the actual performance tests were made with the M95 AP Shot projectile from the 20mm Hispano-Suiza series. It did not meet the Army Air Force's requirement of penetration of 1.5 inches of face hardened armor at 200 yards. There were 25 rounds loaded originally by Picatinny Arsenal in New Jersey. They had56 g of I.M.R. propellant, which was intended to produce a pressure of 51,400 psi and a muzzsle velocity of 3500 f/s. The rate of fire of the T33 was between 900-1200 rpm.

There was also a T5E1 case that was almost identicalbut was fired from the Colt's Patent Firearms Complany's T24 weapon. The T24 was an adapted copy of their T22 which was based on the German MG 151/20mm weapon. The development effort on the T5E1 and weapon began in November 1944 and was terminated around August of 1945.
Thanks Bob, WOW that sounds like a lot of pressure for a 20mm round.