Although rounds like the 5.56x45mm and 7.62x39mm are well established one occasionally comes across proposals for new intermediate rounds. The Chinese have adopted the 5.8mm and one Russian company is experimenting with a 6mm Unified Machine Gun round that was probably also intended for rifles. Closer to home you'll find suggestions that the 5.56mm should be replaced a larger round, usually 6mm. There is, however, a nearly forgotten intermediate round that is worthy of consideration, the 7mm Intermediate. During the original trails for the NATO standard round, both the UK and Belgium proposed intermediate rounds of 7mm calibre. Both rounds used a bullet of around 140gr, the Belgium example being non-streamlined. For a round that is supersonic for its entire effective range this makes sense, simplifying production and also increasing accuracy and tendency to tumble.
Some loadings had velocities as high as 2749fps (838m/s). Both rounds performed very well but were rejected for logistical/political reasons. Also, many officers were yet to grasp the concept of intermediate rounds. NATO ended up with a 30-06 in a shorter case. The FN 7mm round was used by Venezuela. The dimensions of the 0.280 round suggest that a modern equivalent could be made with the same casings as the M855 round. An examination of the sectional densities of various intermediate rounds proves interesting.
6x45mm SAW XM732
7x44 or 7x49
This would seem to indicate that a 7mm intermediate bullet would have both good flight characteristics and excellent penetration against targets. Being a long projectile for its calibre it is also likely that there will be a strong tendency to tumble. The 6mm round would also perform well, but I prefer the heavier 7mm since it will buck the wind better and be less likely to be deflected by foliage. The larger capacity of the bullet will also be useful for tracer and incendiary rounds. The 7x44mm round was 64mm long overall, about the same length as the 6mm SAW and about 11mm longer than the 5.56mm round. Whether such a round could be made to fit existing magazines and actions I don't know. A shortened round, using a 110-125gr bullet would still have a sectional density of between 0.2 and 0.228. A more prudent idea may be to lighten and scale down full power weapons like the FAL, Stoner SR-25, AR-10B and G3. Stoner SR-25s with 16" barrels are already available. This would give a new weapon that is familar to those trained on M16s and has a high number of components in common with both the M16 and the Designated Marksman's Rifle.