You don't always have to copy the solution as long as the problem is solved.
What will not having an GPMG but only SAW and HMG mean tactically? SAW should cover distances up to 500m, beyond that maybe HMG is better than GPMG. Current wars have often been against unarmored insurgents. If the enemy drives around in MRAP a HMG could be more useful.
Just like some countries have rockets in squads while others have weapon squads in platoons. If I'm not mistaken 5.8mm is designed to out penetrate 7.62x54r out to 800m
This source thinks that 50cal gives best suppression per KG ammo.
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Your logic is kind of faulty. I can kill a a mosquito with a baseball bat, and the problem is solved right? Yes, it would be, but doesn't make it the best option. So if you try to deploy HMG in a way like GPMG, then it is wasting the range advantage.
You also mention both unarmoured and MRAPs, what are you planning to face? Both? Unarmored insurgents probably aren’t driving MRAPs, so which is it? Against unarmoured targets 12.7mm is overkill.
Even if 12.7mm is a more effective weapon per weight, carrying less ammo means you are putting more strain on the logistics chain, shorter patrols etc. Also it would be a disadvantage in an extended firefight where you are trying to maximize bullets down range.
Weapon bulkiness is also a real issue. You are going to have a long barrel no matter the weight, that makes it an issue moving in any narrow spaces, urban or wooded areas. Long barrel also means you have to use bipod/tripod to be most effective. Also, in case of mechanized infantry, you are taking up space in the vehicle which already is contending with soldiers with increasing gear demands like body armour, digital gear, etc.
Your interpretation of the data isn't really optimal either. While 12.7mm might look like the best, 7.62 rounds are already reaching close to 90% suppression (by their measure) at typical infantry engagement ranges (400m and under), while 5.56mm is not. So you would trade off twice the suppression time for those marginal gains.
But look at the other thing your chart is telling you. You are getting a lot of performance from 338 rounds without doubling the ammo volume. Is it any surprise that the next-gen GPMG is looking at that round?
It's funny, you can do all these academic studies and research on the net, but most of this is quite obvious even with a few months of basic infantry training.
