As with heavy tanks, prospective Soviet heavy SPGs were well protected vehicles with long 152 mm guns. The first requirements for these vehicles were ready in 1945, but work began only a year later. They were designed on the chassis of the Object 260 (IS-7) and Object 701 (IS-4) tanks.
The SPG on the IS-4 chassis, indexed Object 715, was meant to use the 152 mm M31 gun developed by factory #172, with identical ballistics to the 152 mm high power Br-2 gun. The same gun was planned fro the Kirov factory's SPG. It is unclear what it was called. Some sources say Object 261, others say Object 263.
Later, factory #172's design bureau designed an even more powerful gun, indexed M48. The design was similar to the M31 and even had a similar muzzle brake, but the muzzle velocity was increased to 1000 m/s. Such a powerful weapon could destroy any enemy tank or pillbox without a problem. The semi-open casemate Object 262 SPG was planned with the same gun.
The main obstacle for these plans was the delays of the IS-7 project and problems with mass production of the IS-4. The last activity on these projects happened in 1947, after which they were put on hold until better days, but those days never came.
There was another reason that development of the Object 268 dragged on that is not accounted for by some researchers. The gun that would be installed on the SPG was still being designed. Meanwhile, the factory #172 staff did not sit still. After their M62 122 mm gun which was used on the Object 752 and Object 777 tanks, the designers from Perm started working on 152 mm guns in early 1954.
Seven years passed since the design of the M53, a modified version of which was going to go into the Object 268, and artillery did not sit still. A new 152 mm gun, indexed M64, became available. Its muzzle velocity was almost the same as on the M53 (750 m/s), but the length of the barrel was much shorter. Since the fighting compartment of the T-10 and Object 268 were positioned in roughly the same place, this was very important. To compare, the modified M53 had an overall length from the center of the casemate to the tip of the muzzle brake of 5845 mm, while the M64's length was 4203 mm. The overhang of this new gun was only 2185 mm.