Question is, are we going for 7.62x39 just because we already have it and the logistical transition (at least for RR) could be cheaper? Or because the x39 has genuine advantages in the kind of operations RR conducts, which makes it better than a 5.56x45 (.223) or 7.62x51 (.308)?
Did RR try out 308 to entertain the idea of whether they can look at throwing away the x39?
My guess is that RR is not ready to let the x39 go. And chances are, Assam Rifles won't be, either. But it needs to be seen if a modern .308/.223 rifle can't be a good replacement for all AKs in the Army. Because let's face it - the only experience the Army has at large with a 5.56 AR is the INSAS and the only .308 experience is with the older SLRs...neither of which are full-auto rifles. I'm not surprised if some units would prefer the AK over them.
But a modern assault rifle (Excalibur for 5.56 and a foreign 308 like the Galil ACE or Tavor 7AR) is a completely different deal. Is it wise for RR/AR to stick with x39 rifles (albeit could be modernized AKs) over these beasts and all the logistical & economic benefits it could bring?
As I said, I wouldn't call it a nightmare if Army has 3 calibers of assault rifles as standard-issue. But it does raise the question if such a diversity in logistics is actually warranted or not?
7.62x51 is not fit for firing in close combat situations. In close combat situations where mobility is required, the heavier round will reduce the ammunition that is carryable and also increase recoil. But it is ideal where ammunition shortage is not a problem like in border outposts
7.62x39 is good for close combat with obstacles like forest, urban areas etc. The round penetrates barriers more easily whereas 5.56x45 loses most of its energy on even hitting a small branch.
5.56x45 is good for combat in open ranges as the round has flatter trajectory and lower recoil.
7.62x51 is unlikely to be a round that is carried by mobile soldiers but as LMG only. In this manner, the rounds required will be 3 but in effect only 2 when needed in mobility