If All AK-47s Had Been . . .

[stanc]

Let me put it a different way: if the Soviets had developed a lightweight belt-fed MG in 6.5mm Grendelski they probably wouldn't have bothered with a lightweight belt-fed portable MG in 7.62x54R like the PKM. A heavier support and vehicle weapon, yes, but not for dismounted infantry. The ballistics are not quite as good, but they aren't far behind and the substantial weight and recoil reductions would be well worth having.

Of course those characteristics would be nice to have. But, at the price of developing and fielding two new (and completely different) machine guns, along with the attendant logistical and training issues? I can't say I'd rate that as a probable event, given the Soviets' post-WWII trend to simplify matters in regard to military small arms.

However, I can't say you're necessarily wrong. The Russians would've started out with a lightweight, belt-fed, 6.5mm RPD, which would later have been replaced by a magazine-fed, 6.5mm RPK. Would they then have turned around and developed another belt-fed 6.5mm LMG? IMO, there are too many variables to give an answer with any degree of certainty.

Raising the RPD in its existing (7.62x39) form is a straw man argument. Of course it isn't used, because the long-range ballistics are rubbish and the PKM is of comparable weight. A RPD/PKM equivalent designed around a 6.5mm Grendelski would be a very different matter.

You're mistaken about it being a straw man argument. A 6.5mm RPD would've been produced in the same numbers as the 7.62mm RPD, before being superseded by a 6.5mm RPK. Therefore, it seems reasonable to think that the obsoleted 6.5mm RPDs would have achieved the same worldwide distribution as did the 7.62mm versions. It was a finite quantity, and likely relatively few made their way to Afghanistan.

If the RPD sees little use in Afghan combat, I'd say it's because of the small numbers in theatre, not due to poor long range ballistics. Remember, the AK has even worse long range ability, but is by far the most numerous gun used by the insurgents.