Hint: 7.62 NATO.
Just change the M-4 uppers.
Sorry, that is Not an Option.
Neither Rounds or Magazines for the 7.62x51 fit in an AR-15 Lower.
So Options would be M14 or FAL or the SCAR.
7.62x51 will not fit in a m4 lower, it is to long. AR10 however....
“Hint: 7.62 NATO.
Just change the M-4 uppers”
I believe that it is possible to convert the M-16/M-4 to 6.5 or 6.8mm rounds by changing uppers/barrels, but to get a 7.62 Nato you would have to go to Eugene Stoner’s AR-10 design......
The Nato round is much longer than the M-16/M-4’s magazine/magazine well could accommadate.....