I have never seen that rifle before, was it full auto?
Of course, the whole purpose of an assault rifle was to fill the roles of submachineguns, but give them greater power and range. The role of submachineguns at the time of ww2 was primarily in storming and assaulting positions where close range engagements were expected. Like urban combat.
Firing full auto at targets at relatively close range.
Tactics developed in WW1 were similar and the likely origin of later tactics. The whole "walking fire" while advancing and taking trenches. Full auto fire intended to be "walked" into the enemy while advancing.
So "assault rifle" calibers were adopted, which gave up the power and range of a typical battle rifle, but were more controllable while fired in full auto.
If never intended to be fired in the full auto role they would have stuck with the more powerful calibers of typical battle rifles.
A semi-auto "assault rifle" would have seemed pointless and actually worse than a semi-auto battle rifle to them at the time. Like going backwards.
The ONLY reason the US went to 5.56 was that it was somewhat controllable in full auto fire
Exactly. They chose a round that was less powerful but was controllable in full auto fire.
They do not use them in the original intended role today.
Full auto fire was intended to be the standard mode of operation, and as a result a less powerful caliber that allowed a higher rate of accurate fire was adopted.
They have since retained the rifle (with updates) and cartridge for other reasons unrelated to the original reasons for adoption.
At the time if they had intended for soldiers to operate in semi-auto fire most of the time they would not have gone down to a fast .22 caliber weapon.
Compare that to the PDW type weapons they have come up with more recently. Similar concept. Even less powerful low recoiling rounds that give up stopping power per round but are designed to compensate with a very high rate of fire. (Which have since proven unpopular as obviously the 5.56 rifles already used accomplish almost the same thing with even greater power per round, and require no new purchases, or additional logistical concerns.)