From the Ammo section of my autocannon history book:
20 × 158RB T5
This massive cartridge was developed in the USA in the late 1940s for the unsuccessful T33 aircraft gun. At that time the USAF was concerned to achieve the highest possible muzzle velocity to maximize the chance of hitting enemy aircraft, but this gun and ammunition were dropped by the early 1950s in favour of the more efficient weapons designed around the 20 × 102. The brass cartridge case (designated 20mm T5) is bottlenecked and has a rebated rim. It contains 58g of propellant. Projectiles, which have brass driving bands, are secured in the case by a crimping groove in the case neck. Overall length of the complete round is 230mm and the weight is 375g with a 130g M99 TP projectile. Calculated muzzle velocity was 1,150m/s at a chamber pressure of 338Mpa (maximum 380Mpa). In the Colt T24, 56g of 4,879 I.M.R. powder generated 1,070m/s, at a pressure of 355Mpa.
Which book of yours is that Tony? Is that the one being released soon? Sounds fantastic!
I’ve got a somewhat related 20x135mm T7 cartridge, good to know this also exists, never heard of it before!
Yes, that’s my baby!
Another ammo extract:
20 × 135 MAUSER MG 213/20
This cartridge was developed for the Luftwaffe’s ‘million point gun’ project, for an aircraft gun (designated MG 213) which would achieve a muzzle velocity of 1,000m/s and a rate of fire of 1,000rpm. Various types of gun mechanism were tried, the successful one being the Mauser MG 213C. This was the original modern revolver cannon, first developed around this high-velocity round before attention switched to a 30mm version. The two versions were then designated MG 213/20 and MG 213/30. A few test prototypes in both calibres were made by the end of World War II.
The large, steel 20 × 135 cartridge case is rimless and bottlenecked, electric primed, and weighs 154g. The first version of the cartridge for the MG 213A has a tapered case, but the MG 213C is more cylindrical. Projectiles, which have iron driving bands, are secured in the case by a crimping groove in the case neck. Overall length of the complete round is 199.8mm and the weight is c.340g. The standard HE-T projectile weighs 117g (the tracer burned to 1,250 m) and 63g of Nz.R.P. propellant gave a muzzle velocity of 1,000m/s. The Unterlüss report mentions three other loads:
MX-Geschoss, using the lengthened version of the M-Geschoss (also used in the 20 × 82): weight 104g; HE weight 24.5g; MV 1,000m/s.
Incendiary-tracer (by Hasag): weight 115g; MV 960m/s.
Incendiary (Brandgranate 44): weight 107g; MV 990m/s.
After the war, an example of the MG 213/20 arrived in the USA where it was carefully analysed under the designation T74, and various modifications made. The 20 × 135 ammunition was retained for these analyses, but made with brass rather than steel cases (designated T7). It was probably also used in the experimental ARF T33E3 gun. The loading for this ammunition reportedly used a conventional 104g projectile (T61E1 – a shortened version of the 20 × 110 Hispano M99 TP projectile) at an MV of 1,150m/s. Chamber pressure was 338 MPa.
Fede, excellent! Is there more info on the links or better images?
Cant wait tony! Sounds amazing already
I can NOT even come CLOSE to doing the math, but that sounds like those projectiles were just about nose-to-tail!
Math of 1000 rpm and Vo 1000 m/s tells us the projectiles are 62.5 m apart (omitting velocity decrease).
Much closer are 20mm M61.