The M73 7.62 NATO tank gun and its big brother, the M85 .50 caliber tank gun, were introduced on the M60 tank. Both guns were an attempt by the Army to get a short receiver gun for its armored vehicles that were using both the .30 caliber M37 and M2 .50 caliber machine guns based on the Browning design. The Army designed the M85 to have dual modes of fire; a slow rate for ground targets and a fast rate for air targets. in reality, neither the M73 or M85 worked reliably.
The Army redesigned the M73 and issued it as the M219, but it didn't work well either. Many parts between the M73 and M219 did not interchange. The M85 never did work (in sand, dust, and unlubricated conditions) and both guns usually broke down after a more than a few bursts to test function. When we gave the Israelis M60 tanks, both the M73 and the M85 were immediately replaced. The M85 by an M2HB .50 Browning on the cupola. The M73 was replaced by the MAG-58 tank gun.
Here's the story from Small Arms Review: http://smallarmsreview.com/display.article.cfm?idarticles=114
They had a transferable M73 and M85 at Knob Creek a few years ago. <6 on registry. As a 19K, I still hated seeing them. Even with TLC, they are as reliable as a Chautchaut.
When we gave the Israelis M60 tanks, both the M73 and the M85 were immediately replaced. The M85 by an M2HB .50 Browning on the cupola. Not entirely. Some of the Israeli M48 and M60 reworks mounted the .50 M2 coaxially above the main gun tube, rather than on an external gun mount or in a cupola. There were two reasons for this. One was that part of the Israeli use of the ,50 is as a backup aiming device should a hit from a HE round shatter the tank's primary optical sighting equipment, especially at night when things are likely up close. Very short bursts or single rounds of tracer are fired until they start to bounce off something in the dark, whereupon the gunner lets fly with HEAT or SABOT.
The other reason was that the Israelis lost the majority of their tank commander casualties in the 1973 war to hits by high explosive rounds on the turret, the impact of which took the commander's cupola off and sent it flying, along with the upper half of the tank commander.
The new Israeli Merkava Mark I through Mark IV continue this practice, as do their rebuilds of the M48 and M60, known in IDF service as Magach
The Israelis generally have a couple of MAG58 *roof guns* mounted at the TC and loader's hatch positions. It helps discourage enemy Sagger and other wire-guided AT missle gunners.
