Moving from 120 to 125mm isn’t really an upgrade. The question is Rifled vs smooth.
Rifled lets you use more HE/ HESH/HEAT conventional shells good for blowing apart barriers. Smoothbore is better tank busting. You get APFDS shells out of it perfect for perforating T72 and the like.
The 120mm on the Arjun is rifled. However is that a killer issue not really. The guns on modern MBT are pretty much interchangeable. You can put a comparable 120mm smooth bore in a T72 Both Poland and Ukraine have. Or a 120mm rifled in a M1 or a125mm smoothbore in a Abrams it’s just that there is not really a need or want for that matter. The biggest issue is the magazines
View attachment 57584
Here we have British 120mm rifled ammo.
Swapping ammo thats hard. Tanks interiors are tight spaces and the type of ammo dictates the type of storage. The 120mm Rifled of the British is two piece ammo like that of 125mm smooth bore.
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Here we have Russian 125mm smooth bore.
The Indian however is more like 120mm Smooth bore NATO a unitary round.
View attachment 57583Indian 120mm rifled
View attachment 57587 Israeli 120mm Smooth bore which is built to NATO standard.
Which results in this conundrum. They should be able to simply drop in a L44, L55 or use the Israeli MG253 and just swap the ammo to Israeli or German or American.
Yet according to Berserk the now want 125mm... Indian T72 and T90 both use 125mm so standardizing makes sense. But Again the Ammo.
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Chinese and Russian 125mm is built as binary ammo this is because they chose the carousel autoloader. Arjun uses 120mm Unitary because manually loaded.
T90 is 125mm Binary.
If they want to standardize They would need to change the ammo storage system to match the binary. Which as the British prove is not easy.
The Challanger 2 basically needs a new turret to change to the L55 gun.
The alternative is establish a totally new 125mm unitary type. In which case WHAT THE BLEEP!!
Now the Russians did experiments with a 125mm unitary in the past but clearly that never came to be used. This would mean that the T90 and T72 in India have one ammo system and the Arjun 125mm would have a second with any unmodified 120mm Arjun a third. Do we see the problem?
India seems like it cant make up its mind so it over complicates things. Most nations pick a tank gun type and only change if it becomes clear its not that effective. This is why you have 125mm And 120mm smooth bore as the big two.
View attachment 57587View attachment 57588
There has been flirtation with 140 or 152mm but no actual production.