From footage that was widely circulated on social media at the time, viewable on twitter, and xcancel, it can be concluded that Iran carried out a ballistic missile test at their Shahroud facilities, which were also hit by Israeli airstrikes during their attack on Iran in October, in which a missile casting building/warehouse was hit, which was responsible for manufacturing the casings for Iran's solid fueled Medium Range Ballistic Missiles (MRBMs). So the facility is still operational despite the damage from the airstrikes, viewable below.

As for what missile was tested, there has been much speculation, including that of an Inter-Continental Ballistic Missile (ICBM) test, even by Iranian member of parliament Ahmad Naderi, who also called for the testing of nuclear weapons. I doubt that, and rather think the test is being done in preparation for a future attack on Israel. As for what I believe was likely tested, my guess would be the Khorramshahr-4/Kheibar MRBM.

So what is the Khorramshahr-4 MRBM? It is part of Iran's more advanced Khoramshahr series of liquid fueled ballistic missiles, using a hypergolic (self igniting) propellant of unsymmetric dimethylhydrazine (UDMH) as the fuel, and dinitrogen tetroxide as the oxidiser. This is a significant improvement over the use of kerosene compounds as fuel, and nitric acid mixed with 27% dinitrogen tetroxide as the oxidiser, which Iran's Scud derived designs used as propellant.

The Khorramshahr-4 is a design that takes inspiration mainly from the Soviet R-27 Zyb Submarine Launched Ballistic Missile (SLBM), the North Korean Hwasong-10/BM-25 Intermediate Range Ballistic Missile (IRBM) which is a North Korean version of the R-27 Zyb, and the North Korean Hwasong-12 IRBM. The story starts with Iran purchasing and modifying a Hwasong-10 IRBM for their own use, making the motor more fuel efficient, to allow for the use of a bigger warhead on the missile with smaller fuel tanks. The Khorramshahr-4 was unveiled in 2023, with a 1500kg warhead and 2000km maximum range. This is a fairly large warhead for an IRBM. However, there is speculation that with a lighter warhead, the range could be extended well past this, form anywhere between 3700-6000km. Tests of the Hwasong-12 IRBM have shown that a range of 4500km is possible. Iran could also be understating the range to keep relations with Europe and East Asia more amicable.

So what are the positives of this weapon, when compared to the solid fueled Fattah-1 and Kheibar Shekan IRBMs with Maneuverable Re-entry Vehicles (MaRVs) Iran used in their attack on October 1st? The first advantage is the size of the warhead, with the Khorramshahr-4 warhead weighing three times as much as those used previously. With more explosive power, less accuracy is needed. This also enables the use of submunitions and cluster bombs in the warhead, which is a capability of the Khorramshahr-4 that Iran has already made public. The 4 meter tall warhead has plenty of space for that, as pictured below. This also means accuracy is less important, as you can hit a lot more, even if overall penetration and damage goes down. Think of it as a shotgun vs sniper rifle.

As for another advantage, the guidance systems of the Khorramshahr-4 are said to be resistant to jamming. According to Iranian officials, it makes use of a mid course phase guidance correction system that operates when the missile is exoatmospheric, so it is not reliant on guidance in the terminal phase which can be jammed, such as GPS guidance. My guess on that one would be that the Khorramshahr-4 makes use of a celestial-inertial navigation system, similar to the Galileo system used in modern French SLBMs. This involves using a camera to pick up on the position of the stars, and correct errors with the gyroscopes in the Inertial system. Navigating using the stars. A lot of older SLBMs used this, as without a fixed launch point and no GPS, it was a good way to improve accuracy. The Khorramshahr-4 is derived from the R-27 Zyb SLBM after all.

As for disadvantages, for one it's a traditional ballistic missiles, and the re-entry vehicle/warhead is not fully maneuverable, there are no aerodynamic fins to change course. This makes such a weapon easier to intercept, even with its fast speed, of Mach 16 while exoatmospheric, and Mach 8 while in the atmosphere. It's just a normal, parabolic ballistic trajectory. It won't be able to perform the skip and glide trajectories that MaRV capable missiles can.

Another disadvantage could be accuracy. The original R-27 Zyb SLBM did not need to be very accurate, it carried a 1 Megaton nuclear warhead, about 62 times more powerful than the bomb dropped on Hiroshima. It had a mean error radius of over a mile/1.6km. However, this is a problem for the conventional explosive Khorramshahr-4, and Iran has made attempts to improve the accuracy, similar to the DPRK. They have mounted four veneer rocket motors to the rear of the re-entry vehicle/warhead, similar to the Hwasong-12, to help steer and guide the warhead and improve its accuracy. Here's a video showing the operation of the veneer rocket motors. Seriously worth watching. While these can't make full manouvers, they will allow for course correction. Iran claims a sub 30m mean error radius in testing. I doubt that, and think a 100m mean error radius would actually be very good under real world conditions. Iran's attack on Nevatim had a 900m mean error radius, likely due to GNSS jamming and errors with the inertial guidance systems causing overshoots.

Overall, the missile test, Ansarallah launching Palestine-2 missiles at targets in Negev desert to probe Israeli air defenses, and the speculated use of such weapons like the Khorramshahr-4, shows that Iran is testing the waters for a potential retaliatory strike against Israel. An attack with the intent to hit more targets than the last, while still remaining efficient, and without firing thousands of missiles.