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Being an ex tanker, light and heavy, I'm intrigued to actually know the reason for the .50 mounted above the main gun, is it just like an internal co-ax MG and for supporting fire ? I take it that it's slaved to the main gun controls ? Or not ??
I seen it on nearly all IDF armour over the years, but never really thought about the usage of it till now.
The answer depends on the time frame of your reference. The .50 Cal fixed to the barrell, as well as an additional MG on the turret, started to appear on IDF tanks after the Yom Kippur War.
During that war, the IDF lost large number of tanks to anti-tank missiles launched from well concealed, almost undetectable positions. The additional MGs were supposed to provide more suppressing fire during an assault. That probably wouldn't destroy them, but at least would keep the heads of those AT teams down.
During the '82 war those heavy MGs were probably handy during urban warfare. Interestingly, most of the Magachs and Shots that participated in the campaign did have .50s, most of the Merkavas (maybe all) did not. Some M109s also had this feature.
In the '90s, when I've served on the Mk.2B (and for a short period on the D in Lebanon), something must've changed, because we only used the .50 cal on certain trainings to simulate tank shells up to 1.500m. We always removed them before operational deployment in Lebanon, the West Bank and Gaza.
Mk.3s in Lebanon during the '90s. No .50 cal.
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As a matter of fact, in case of an all out war with Syrians on the Golan Heights (the main focus of our training that time), we would immediately ditch the .50 cals. We've never been told why, but I guess they would've been useless anyway in the type of defensive war we were supposed to fight.
During the second Intifada (from late 2000 until about 2005 and occasionally since then) the IDF has found itself involved in an asymmetric, mostly urban warfare. In such scenarios the use of the main gun is limited, so I guess the additional firepower of the .50 cal as a beefier coax is appreciated. The .50 cal seems to be a more or less permanent fixture on operational IDF tanks ever since.