By the middle 1944 it become clear that existed Soviet antitank weapons insufficient for successful combat with the newest German tanks. It was needed to to increase the firepower of the Soviet tanks and antitank guns.
Question was solved by developing a new self-propelled gun armed with a very powerful 100 mm gun with ballistic, similar to the B-34 Naval AA Gun. By December 1943, all drafts of the further vehicle were sent to the NKTP and, simultaneously, to the Department of the Self-Propelled Artillery. On December 28, 1943, the NKTP ordered the Uralmash to arm the new vehicle with the 100 mm gun:
"1. By 15.01.44, a new medium tank destroyer on base of the T-34 must be developed and being armed with the 100 mm S-34 tank gun; 2. By 20.02.44, to build and test that vehicle. Not later then 25.01.44, the tank gun must being received from the factory #92; 3. Not later then 25.02.44, send the new vehicle to the Governmental trials."
Yes, the terms of this order were very demanding. After the line-drawings of the S-34 gun had been obtained, it became clear that this weapon could not be mounted on the tank destroyer, due to the width of the gun which made an unacceptable limitation on traverse and did not allow room for the driver's hatch on the front of the hull. However, the modification could not be achieved in any case within the time-frame of the order.
That entailed the changes in constructions of welding and assembling benches. It was need also to use a torsion bar suspension, move on 100 mm left the driver's station and all the controls, to broaden the whole compartment which would increase the weight on 3.5 tones. However, the TsAKB took uncompromising position and was persisting to install its S-34 gun.
The Uralmash asked design bureau of Factory #9 to develop a new 100 mm Tank Gun, and work (led by F.F.Petrov) was finished by middle 1944. The new gun was based on the 100 mm B-34 Naval AA Gun. In fact, the basic model of the new gun was named D-10, while it's tank variant named D-10T and it's SP gun variant named D-10S. Of course, the D-10S and D-10T had the same ballistic and had only a minor differences concernind their mounts. To be precisious, I must say that the first time those guns did not distinguished in documents and all of them named simply D-10. The additional "T" and "S" letters appeared later, somewhere in 1950's.
The D-10 was lighter then the S-34 and could been installed in existing hull without any serious alterations. On March 3, 1944, the very first prototype with the D-10S passed a local program of tests on the Uralmash factory which consisted of 30 shots and 150-km race.
After that, the vehicle was sent to the Governmental trials, which took place near Gorokhovets town. From 9 to 27 March of 1944, vehicle being tested. During those tests it fired 1040 shots and run a 864-km race. New vehicle was named SU-100 and recommended for mass manufacturing after some minor improvements must been done. On April 14, 1944, the Uralmash was ordered to begin a mass production of the SU-100. The TsAKB however, demanded (again!) to arm the vehicle with its S-34 gun. After a long and heated debates, the GKO ordered the TsAKB to modify existing variant of the S-34 to reduce it's wide dimensions and, after that, mount it in the SU-100. All those works were done by the Factory #9.
Those works were pointed to shorten the gun-cradle on 160 mm, to make a new pivots, to make a new traversing mechanism and to do some other improvements.
New modification of the vehicle received the SU-100-2 designation. From 24 to 28 June, 1944, both vehicles (SU-100 and SU-100-2) were tested. After 923 shots and 250-km race SU-100-2 was recognized as worse and did not recommend for further development and production. The SU-100 was recognized as successful and "it's tactical and technical characteristics allow it to hit any modern AFV at ranges of: - 1500 metres both Panther and Tiger when hit any part of those tanks; - 2000 metres for Fedrinand's sides" (Source: The Order #6131. July 3, 1944)
A new commander's cupola was installed on the roof as well as MK-IV observation sights, also a pair of the new ventilators were mounted to combat with asphyxiating powder gases. Generally, the 72% of the SU-100's parts were borrowed from original T-34, 4% taken from the SU-122, 7.5% - from SU-85 and only 16.5% parts were new. The 100 mm long barreled (56 clb) D-10S gun was supplied by 18 armor piercing rounds BR-412B and by 15 fragmentation/HE rounds OF-412. Having a 895 m/s muzzle velocity, the BR-412B with a blunt ballistic cap was able to penetrate about 100 mm armor sloped at 60° at range of 1500 metres.
Notwithstanding, there were a known problems with the D-10S, its fine tuning continued till the summer 1944. Moreover, there were large problems with mass production of the BR-412B, the technological process was too complex and final production was of low quality resulting a reduced armor penetration. Thus, the mass production of the SU-100 was delayed until problems with ammunition won't be solved. They were resolved by November-December 1944. Until that date, the mass production of the SU-85M was launched instead. In December 1944, the manufacturing of the SU-85M was cancelled and mass production of the SU-100 was started instead.
Having an excellent firepower and maneuverable, this tank destroyer was able to penetrate a frontal armor of any German AFV from a quite long range. It's AP shell could penetrate 125 mm vertical armor from 2000 metres, while from distance of 1000 metre it could penetrate most of German AFVs through. In spite of many guessings, SU-100 did have neither HEAT nor HVAP ammunition. These ammo were developed only after the end of WWII.
The SU-100 had a "classical" scheme. At the front there was a combat compartment with the D-10S mounted in it; at the rear - the engine's and the transmission compartment, the last consisted of the friction clutch, the five-speed gearbox and two side friction clutches. Plus, two fuel tanks and pair of air filters were located in the transmission compartment. Total capacity of all internal fuel tanks was 400 liters that provided the SU-100 with 310 km range. Tank controls, armament, ammunition, radio, and frontal fuel tanks were set up in the combat compartment. The SU-100's driving gear was totally borrowed from the T-34.
The D-10S gun was equipped with two sights: the telescopic and the panoramic. Ammunition consisted of 33 cartridge loading shots. The gun's mantlet was fastened by eight bolts. The practical rate of fire was about 5-6 shorts per minute.
From December 1944, mass production of SU-100 begun. Till the autumn 1945, UZTM has manufactured 2495 of SU-100's.
In summer 1944, the experimental self-propelled gun ESU-100 was developed. It had an electric transmission. Due to some reasons ESU-100's project was abandoned (mostly due to low reliability). The mass production of the SU-100 continued to March 1946. Some sources claims that in 1947 the production of SU-100 was resumed and additional 198 vehicles were manufactured. In 1950th SU-100's were manufacturing in Czechoslovakia after the USSR's license.
From December 1944, some Soviet regiments and brigades of the medium self-propelled artillery being rearmed with SU-100's. Each of that regiments consisted of four batteries of 5 vehicles plus one commander's SU-100 or T-34 per battery. A self-artillery brigade consisted of 65 SU-100's. On January 8, 1945, SU-100's were used for the first time (that was in Hungary). The mass usage of the SU-100s was during infamous German counter-offensive at Balaton lake (Hungary, March 1945). In other places SU-100s used ocasuonally. Till the end of war, total 3037 SU-100's were built. Th SU-100 served in USSR up to the end of 1970's, in most of Eastern countries (members of Warsaw Pact), in Asia, Africa and Latin America. This vehicle took part in military conflicts on Middle East, in Angola etc.
In autumn 1944, due to delaying the mass production of the SU-100, the Uralmash's Design Bureau offered a project of a tank destroyer based on the SU-100. Like the SU-85M, this vehicle was the common SU-100 but armed with a 122 mm Tank Gun D-25S. Such modernisation was possible due to unified fastening of both guns.
The very first trials of the SU-122P conducted in October 1944, after that the vehicle was sent to the Governmental trials which were passed as well. By the results of both trials, the SU-122P was recommended for service, but it didn't mostly because of slow rate of fire.