Testing, testing...

[stanc]

I don't agree that the task is to replace the 5.56mm rather than the 7.62mm. My perception of the UK view is that the weight of 7.62mm belted MG ammo is a bigger issue for dismounted troops than any lack of performance of the 5.56mm.

Hmm. That's just the reverse of what I got from your "bite the bullet" article and presentations.

IMO, 6.5mm Grendel is currently the closest to the ideal performance of any current cartridge.

That's what I used to think, too. However, with the switch from SAAMI to the Spec II chamber, 6.8 SPC is looking much better than it did. Muzzle velocity with bullets of the same weight and type, 6.8 SPC outdoes 6.5 Grendel by 200-300 fps. Since a new carbine/rifle/LMG need not have the M16 mag well dimensions, neither does cartridge OAL have to be 2.25 inches of the 5.56 round. Let cartridge length be 2.40 inches, and 6.8 SPC can use very streamlined bullets. Combined with the higher velocity, and now 6.8 SPC might not only equal 6.5 Grendel at long range, it might even beat it.

When using 123 grain Lapua it is significantly better than 7.62mm M80 in both wind drift and trajectory at 1,000m - a service ball bullet wouldn't be as good, but should be in the same ballpark.

I'm skeptical of that. The only real data we have to go on is for the 120gr Norma FMJ, which has a 0.428 BC. That's far below the 0.54 BC of the 123gr Scenar. I just don't see the BC of a 120gr FMJ getting above 0.44-0.45, and 110gr is said to be the "sweet spot" for this cartridge -- what's the likely BC for an FMJ of that weight? With a BC in the 0.42-0.45 range, to match 7.62 M80 trajectory and drift, it'd be necessary to have MV in the neighborhood of 2700 fps, and that just ain't gonna happen.