There were early experimentals in .276. There were later experimentals during the period when the 7.62mm was being developed; some were converted to 7.62mm either with chamber inserts or new barrels.
Civilian conversions to other calibres have been done, like .270 and 7.62. An M-1 could be adapted to most any cartridge the same length or shorter than the '06, with the same head diameter. Catch is to get the barrel.