I think that the British and (especially) American armies would need a great deal of convincing of the merits of a calibre as small as 5.56mm NATO. Even after WW2 the British decided that 6.8mm was ideal and the American brass refused to consider anything smaller than the full-power 7.62x51, even with all of their war experience and the example of the StG 44 (and later the AK47) in front of them!

Possibly the Germans wouldn't be happy as well - they were looking at assault rifle rounds pre-WW2, but no smaller than 7mm calibre. Only the Russians would have been happy with the AK47.

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Tony Williams: Military gun and ammunition website