Just guessing, but:
1. Apparently adopting a slightly shorter cartridge (51mm) is easier to engineer than for a longer one (63mm) within the existing receiver in terms of the bolt being able to travel far enough to the rear in order to pick up a new round. The 8 X 57 is right in the middle between the .30-06 and 7.62 X 51 when discussing length of case. It might be that just that little 6mm could be enough to prevent this while with a shorter case this is not an issue.
2. What is the supply potential for BAR mags vs M1A magazines? With the 8 X 57 version the mags are surplus VZ-26 and 26/30 with no mods required. When this project got off the ground they were plentiful and reasonable. Rick bought a ton ... many of us also bought extras. Now they are much harder to find and less reasonable.
Magazines will be a bigger problem than caliber since Rick stated early on that he did not want to start having to manufacture mags as well as rifles. He provides 2 mags with each rifle. Mags have to be relatively easy to procure and not be cost prohibitive. BAR mags are neither. M1A mags are both (though he still has to mod them slightly).
Just my 2 cents worth from observation.
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