I remember reading in Soldier of Fortune about the development of a version of the recoil action just prior to and during WWII that made .30-06 feel like .22 short rimfire. In fact, one of the prictures depicted the son of the original inventor testifring a man-portable .50 BMG sized autocannon. Unfortunately, there were a series of setbacks to the program:
1: It was Australian in origin, which meant that it had nigh-insurmountable political disadvantage anywhere outside the British Commonwealth. (And plenty of it within it when Britain or Canada and Australia had different parliamentary majorities).
2: It was being worked on in Queensland, which meant a serious disruption of R&D after New Guinea was occupied.
3: Australian test prototypes against first the L1A1 and FAL and then Steyr AUG and FAMAS, according to its inventors, won the contest according to any regular standard of accuracy, reliability, and controllability, but test details about ease of cleaning and loading featured notes that didn't match up with the numerical ratings. Furthermore, the inventors implied that the contest was rigged against them by politicans and quartermasters compromsied by Enfield, FN, FA, and Steyr.
Assuming the possible screwdriver that this could have been made to work if critical research hadn't had to be abandoned and restarted, how would it have changed WWII and Post-WWII warfare to have had a weapon that fired battle rifle calibers at full auto with full rage with the accuracy and controlability (and weight range) of an assault rifle?