Though if you start doing CQC work in urban terrain, that 7.62mm round ain't going to be much fun for support troops or "friendlys" chewing it's way through double brick cavity walls with ridiculous ease...
5.56mm offers the best compromise between range, penetrative power, wound effects and firepower (ie: ability to carry sufficient rounds), IMHO. I've fired and used both calibres on exercise (though more extensively with 5.56mm) and greatly prefer 5.56mm.
Getting rounds on target is more important than sheer hitting power. If sheer hitting power were the most important attribute, we'd be using 338 Lapua or some such round.
All types of calibres were trialled against 5.56mm when it was introduced and it generally came out on top. I really don't think a switch to 6.5mm or some other round is going to solve the eternal question of the "best round". A mix of calibres is probably the best option, depending on the available logistical support.
Specwarries are certainly moving towards mixed calibres and don't seem overly interested in a "middle ground" calibre...
Cheers.
I think AD’s post provides a balanced argument in favour of mixed calibres. IMO, if we are going to have mixed calibres, it makes a lot of sense to stick to the two now in service.
The 7.62mm (or .308W in civilian use) was originally adopted by NATO ahead of established .30 calibres like the .30/06 which was in service with US forces and the .303 which served throughout the British Commonwealth. Whilst its ballistic qualities are slightly inferior the 7.62 x 51 had an advantage over the .30/06 in having a shorter, rimless case, which improved ammunition stowage and feeding in magazine fed semi auto and auto weapons. It also enjoyed these advantages over the .303 and was superior in ballistics as well. I'm not sure about the US military but Britain was certainly looking at smaller calibres in the 7mm class. However, the proven success of the .30 led to the adoption of the 7.62mm by NATO as its standard. I think the reason the .30 calibre has been used for so long is that it is probably the largest calibre that the average trained infantryman can shoot reasonably accurately.
A lot of hunters (deer etc) also favour this calibre, though higher velocity cartridges with flatter trajectory have been created by necking down the .30s. For example the .270W and .25/06R are necked down from the .30/06 and the .243W is necked down from the .308W (or 7.62mm x 51). These are all popular for hunting deer but usually when hunting something that can bite or claw back the larger calibre is chosen. Of course there are more powerful choices like the various .338s, .375s, .458s, along with a range of magnum cartridges, but most people just can't handle these.
Shooters hunting thin skinned game (kangaroos for example) have long favoured high powered .22 calibre (5.56mm) cartridges like the .222R, .223R and .22/250. These are easy to shoot and have very flat trajectories which mean little allowance has to be made for drop over normal hunting ranges. As humans are thin skinned it was probably natural that the military looked to these cartridges when it was desired to develop a calibre for semi auto or auto weapons that would be light and which would also enable large quantities of ammunition to be carried. Initially some saw this as a replacement for sub machine guns and carbines, like the M1 carried by US support troops, but in service it eventually supplanted the 7.62mm as the standard issue in the infantry of NATO and other western countries. The calibre adopted by NATO was of course the .223R or 5.56mm. The benefit of the 5.56mm is, IMO, that it is the lightest calibre that has a reasonable chance of bringing down its target with a well placed shot and it is comparatively easy to train people to do that.
Why not stick to the two standard calibres now in use with western forces and match the calibre issued to deploying troops to suit the situation (terrain, environment, etc)? I can't see what would be gained by developing a middle calibre that would be hugely expensive, involve massive logistical issues and probably end up being a jack of all trades but master of none.
Cheers