As I said in that other topic, the major advantage of the 5.56 NATO is generally that it fragments (without being a fragmentation bullet) causing severe internal damage, but not killing outright most of the time (it will kill later on, as removing each shrapnel requires a lot of work, and if it's badly made there will be poisoning), unlike heavier bullets that can go right through, but cause immediate damage.
Hitting vital areas will however, as usual, kill outright, whatever the bullet. But in the battlefield, there is one "fun" fact. If you kill someone, his mates are going to say "Oh! He's dead! Resume shooting!" whereas if you wound someone, that person will go loud, one or more of his mates will go "Don't worry dude! We'll save you, you hear?". This means by hitting one guys, you actually reduce the fire power coming at you by 2 or 3 shooters. Hitting is more important than killing, and 5.56 are generally more accurate.
The 5.56s are however more prone to being affected by environmental hazards (Wind, leaves, ricochets, etc... ), reducing their efficiency in dense terrain. Firing through leaves reduces the bullet speed or alters the path, city fighting can cause friendly fire or collateral damage due to ricochet, as well as hitting trees.
Based on my FAMAS knowledge: a bullet can still go through a standard army helmet at approximately 200m range at the right angle, where the maximum engagement range is (for old versions) 300m, or (for more recent ones) 400m. For bigger ranges, you use dedicated weapons that have heavier bullets.
The small calibres biggest advantage is that you can carry twice as much ammunition when fielding 5.56 ammo than when fielding 7.62 (nato) ammunition for roughly the same weight. This means you can actually shoot more, for prolonged engagements.
The ideal would be to have a intermediate ammunition (the 6.5 GRENDEL could be good) for both hitting power and ammunition load. Combining weapon calibres in a squad for different roles/ranges woks, but puts a strain on logistics, and if the guys with one type of ammo run out, then well... You can't share.
My opinion is that for grunts, the main weapon has to be the 5.56, but switching for urban combat to a safer, heavier calibre. Developing a 7.62 bullpup assault rifle might be a smart move (especially with an ejection system like the F2000 or the [SMG] P90, to ignore the only field disadvantage of bullpups) for a dedicated urban weapon.
This is simply because the distance of engagements in general is long where the 5.56 has a slight edge, while urban combat has so many specifics that it would be smart to have weapons specifically for it.
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