While working on a new tank, Kartsev was looking for a more powerful tank gun. The 100 mm D-10T and D-54 tank guns had a fierce opponent in the form of the British L7A1 tank gun. The Soviets decided to "recaliber" the already existing 100 mm D-54TS tank gun. The modifications done to the gun included removing the rifling of the gun, reducing the profile of the bullet chamber, removing the muzzle brake, lengthening the gun tube, adding an automatic cartridge-case ejector, and adding a bore evacuator in the middle of the gun tube (as opposed to the D-45TS tank gun, which had a bore evacuator in the base of the gun tube). The new 115 mm tank gun was designated U-5TS "Molot" Rapira; it was the first smoothbore tank gun. When it went into serial production, it received the designation 2A20. It was put in trials against the D-10TS tank gun, which armed the T-54B as well as some T-55 and T-55A medium tanks. These trials showed that the under-caliber projectiles fired from the U-5TS had a nearly 200 m/sec higher muzzle velocity. It became apparent that the maximum range of the new tank gun was almost double that of the D-10TS. The only serious drawback of the U-5TS tank gun was the fact that it was not as accurate as the D-10TS, because of the lack of rifling. However, the greater range of the gun and its extremely high muzzle velocity made the poor accuracy less of an issue.