NonCredibleDefense

While the Ordnance Department made a lot of terrible decisions, I don't think the development of 5.56mm as a caliber was delayed much by them. Armalite was doing it's thing and dealing with Remington to fine tune their design. The US ended up adopting a 5.56mm rifle only 5 years after the Soviets had gotten the AKM production up and running. 5.56mm is a lot better for general use than 7.62x39mm so the delay may have been worth not just making a ballistically crude round as the 7.62x39mm.

We can go back and forth on if the .280 British should have been adopted, but it did have legitimate accuracy problems in both US and UK testing that at least make it's non-adoption understandable.

While I can somewhat excuse the adoption of 7.62x51mm in the post-war, what I find indefensible is the M14 itself. The idea of reusing the M1 tooling to cut costs should have been obvious nonsense. The FN FAL was a much better design for a service rifle. Add to this the idea of removing both SMGs and squad automatic weapons from service in favor of using an M14 in their place was clearly a doomed endeavor.

Though if the US hadn't adopted something as awful as the M14 maybe the Air Force wouldn't have bought AR-15s and we never get the timeline where the AR-15 wins, so. Eh.