Once the US gets bogged down in the Vietnam war they'll still switch to the M16 and it's possible that would be in .280 but I think it more likely the US would opt for 5.56 simply because it was invented in the US.
Ok, .276 Pedersen then. It's pretty ballistically similar to .280, and Made In America. One of my personal fanboisms is the US adopting the Garand in .276 Pedersen as God and the Infantry Board intended. In addition to making the Garand
even more awesome, we might still be using the cartridge today. At the muzzle it's not
terribly ballistically different than a modern 6.5mm Grendel- I'm not sure about the performance at range, though. Anyway, have the US adopt the .276 in some capacity and that could easily be a universally-adopted cartridge for NATO that would be very appropriate for the EM-2.
Ew. The charging handle reciprocates. Right next to your fingers. -2 style points. Also, using a safety like the Garand's is worth at least another -1.
Still, yes, I always thought the EM-2 was a missed opportunity, and a damned interesting rifle. It
looks sexy, if nothing else- which is difficult for a bullpup.
Incidentally, bullpups are not the revolutionary advancement that a lot of people think they are. They certainly have their advantages, but I would be hard pressed to say they are superior to a conventionally laid-out rifle. Changing magazines quickly takes a LOT of practice, especially if you don't want to unshoulder the rifle. The long/massive trigger linkages tend to lead to rough triggers. And unless it has the added complexity of a downward- or forward-ejecting action you can't shoot around corners to the offhand side easily in MOUT (what was above called FISH), lest you find the brass implanted into your face.