IFVs are great for adding firepower to infantry units. And the standard arms race between gunmakers and armormakers has the added complication that the IFV has to carry some infantry to actually do its job. So, unlike tanks, it gets progressively harder to increase the gun caliber in an IFV if you actually want to carry an appreciable number of shells. One such answer can be found with a little inspiration from wildcat cartridge makers. We can take our regular autocannon round, neck the cartridge out so that it’s a straight-walled cylinder, replace the barrel with a bigger one, and get a more powerful round without sacrificing ammo capacity. The most potent such example was proposed in the 80s, and is known as the 50mm Supershot.
The base round is the 35x228mm autocannon round used in the Bushmaster III chain gun. Figuring that one couldn’t go much larger without serious complications (cf. the CV9040 with the 40mm Bofors and its pitiful ready capacity of 24 rounds), ammo designers in the 80s decided to try to make the 35mm round bigger. Necking it out gives you a diameter of 50mm, so that’s the caliber they went with. The resulting round is somewhat longer though to get the power right, since the rounds are semi-telescoped (i.e. the propellant doesn’t totally surround the round). Way cool. The 50mm Supershot gives the same propellant capacity as the 40mm Bofors round, which is a big plus. It’d be a hard-hitting KE round, and would have the capability to launch a significant amount of high explosive.
Development of the 50mm Supershot stopped with the end of the cold war. That hasn’t stopped us before though. What’s a bigger problem here is actually market forces. First, 35mm is not a very popular cannon round, which means there’s a much smaller pool of potential users to pool development costs and production runs amongst. 35mm is a big round, so those who favor the suppression fires type armaments are going to look elsewhere. Where the CV9030 holds 160 ready rounds of 30x173mm, the CV9035 holds only 70 ready rounds of 35mm. Even those nations who have gone with the 35mm (e.g. the Dutch) are likely to accept that as sufficient for the foreseeable future; the Dutch chose the 35mm as a hedge against uparmored BMP-3s which haven’t materialized, so why would they upgrade further?
The second problem is rival rounds, specifically the 40x255mm CTA. This round is fully telescoped, so the actual cartridge size is 65x220mm. That said, it’s remarkably compact and can be fit efficiently into ammo storage spaces. Because of the shortness of the round and the alternative feed system, you can fit more 40mm CTA rounds than 35mm rounds into a given volume. Plus, the 40mm CTA holds as much propellant as the 50mm Supershot, so you’re not giving up anything in the way of launching power. If we wanted firepower, the 40mm CTA is the way to go. When converting the Bradley to use the 40mm CTA, designers were able to fit 105 ready rounds, which is pretty impressive. If the designers worked with a purpose-designed turret, they could almost certainly fit somewhat more. Finally, the CTA round has already been developed and is entering production and service now, whereas the 50mm Supershot would need some time and money to complete development. On the other hand, we currently field 35mm guns, and more firepower on our IFVs is always a win.
Verdict: Referred to the Borgundy Army Ordnance Board for testing and development