In 1957-58 as a Lt.-Col. I while commanding 38 Medium regiments took part in an exercise called Doaba in Panjab supporting an armored Brigade. At the summing up of the exercise it was brought out by Chief Empire Gen. Kulwant Singh that the 76mm guns on Sherman did not have sufficient fire power to beat enemy armor. 

In 1959-60 when I was commanding the Proof and Experimental Establishment at Balasore I evaluated  the firepower of AMX-13 with 75mm H.V. gun to assess the maker's claim and found it exceptionally good to beat thick armor. I put up as a new idea of my own to the General Staff that we should upgun our Sherman tanks (the wasting assets of the army) with this gun, and requested for a obsolete tank to be given to me for carrying out the modification. A tank was given to me and in 3 months I mounted the 75mm High Velocity gun as my personal project and requested the Director at Armoured corp. General Rajinder Singh  to see a demonstration at my location. The firing was a great success and the idea accepted.

Your observations of its weakness were known but set aside by the USER as I had mounted the gun on the existing turret, no one else in the world succeeded in that by then or later. India did not manufacture a turret by that time. The modification was primarily meant for Anti-Tank Role and this was well played in 1965 war from the hides. For this outstanding example showing ingenuity and skill for up gunning and saving 100 crores of wasting assets of tanks I was given in the list of Honors and Awards by the Chief of the Army Staff, COMMENDATION CARD and subsequently when the ministry of defense introduced cash awards for New Ideas I was given an award of rupees 2000.

The following extracts may be useful to define the capability of  this upgunned tank the name by me to it.

Extract from The Economic Times New Delhi Wednesday 6th September, 1995 page 7 an article by Global Watch/K Subrahmanyam titled “The First War with Pak”
“Third, the Pakistanis had secretly raised a second armoured division while India had only one then... The Pakistani armoured division which was to break through at Khem Karan was totally destroyed by an Indian armoured brigade consisting of one Centurion regiment, one regiment of Sherman Mark IV tanks and another with up-gunned Shermans. The Sherman tanks were of World War II vintage. This happened because of the superior tactical skills of officer and men of the Indian Army. That battle was crucial in the sense if India had lost it the fate of the subcontinent would have changed...The country has to be overly grateful to the officers and men of that armored brigade.”

Extract from The Indian Express New Delhi Friday December 19, 2003 Page 9 “Last Salute to the lion of 1965” (Lt. Gen Joginder Singh Dhillon; 1914-2003, Obituary)
“It is not possible to describe this 17-day war here but the decisive tank battle of Assal Utar, near Khem Karan, on September 10 does bear telling. Indian units hid their Sherman tanks 500 meters apart in a U-shaped formation in tall and unharvested sugarcane fields, and snared the enemy's vastly superior Patton tanks into this ambush, annihilating them to the last tank and deciding the outcome of the war. The destruction of Pakistan's armored pride and the casualties it suffered...”

You may like to add that: 
- Credit for the availability of the up-gunned Shermans goes to the idea and invention of Maj Gen R L Jetley (Retired), for which he was paid merely Rs. 2000 as a CASH Award to kill the dream of Field Marshal Ayub Khan - ON TO DELHI, however credit goes to the General Staff Army for providing him a Sherman Tank to experiment with his New Idea.
- Similar idea to upgun T55 tank with 105mm gun was initiated by me as a student's project at the Institute of Armament Studies when I was the Director of Studies (Army) which was a success, and later passed it on to the army when I was Controller Inspection (Weapons).