Dennis DeMille's original 6.5 was based on the 250 Savage. When Hornady took over they stated the 30 TC parent case was a shorten 308 mostly due to marketing appeal. The 240NMC and the 6mm XC are the same class cartridge from the 22-250 and were direct competition.
In the early days of the 6.5 CM most competitors formed brass from Lapua 22-250 brass. The neck was .025" short but brass quality was much better than the limited available 6.5 CM Hornady.
https://rifleshooter.com/2015/11/lap...-22-250-brass/
The 6.5 Creedmoor uses the 250 Savage as its parent case. Lapua doesn’t make 250 Savage brass, but they do make 22-250 Remington brass (which uses the 250 Savage as its parent case) that I happen to have on hand. Note: some shooter prefer to form 6.5 Creedmoor brass from 308 or 243 Winchester cases.
End result whether formed from 22-250, 250 Savage or a 308 based case the 6.5CM ends up as a 250 Savage length case. The case length would be my primary concern for functioning in the Garand?? The 6.5 CM works well in M1A's & AR10's but they started as a 308 not a 30/06. The 308 functions perfectly in the Garand but how short is too short?
At some point short cases become an issue. The only firsthand experience I have is with 308 lengths bolt guns. Most have no issues feeding 22-250 length cases but most do not tolerate 6mm BR length cases.
My two M1 Garands are in 30/06 but I have barreled around 8 in 308. I do find the 35 Whelens and the 458 Win Mags interesting.