From the original ones from the '60's that I've seen they are standard Rem. Varmint type contours. One barrel had a .820 muzzle diameter and the other one had like .835. Both were 1-10 twist.
I've seen factory Rem. varmint type contours measure from .820 to as big as .850 but I feel the .850 was out of the ordinary. We do a standard .830".
Original barrels were C.M. steel as well. Not S.S.
I don't know why the original M40 barrels were 1-10 twist, when the standard for 7.62 Nato is 1-12 which is the same industry standard for .308win. They probably went with 1-10 because that is the standard twist rate for .30-06 which going into Vietnam was still being used. Most likely if Douglas produced the original barrels for the .30-06 guns being used at the time they just used the same button to make the M40 barrels. Or the other reason is the military just wanted to stay with the 1-10 twist.
1-10 twist possibly to use heavier bullets? Army M24 is 1-11.25 vs USMC M40A1-A3 is 1-12. Doesn't seem to be any standard set.
I would say maybe o.k. the 1-10 twist for heavier bullets but a 1-12 shooting conventional match bullets will stabilize up to 190gr. bullets. The only need for a 1-10 is if your shooting like 200's, 208's, 210's. Doubt they had any factory loaded 200gr. + weight bullets back in the 60's and even now a days I don't know of any factory loaded 200gr+ match type bullets for .308win.
Make sure which M24 weapon system your looking at also. The M24 in .308win. might be spec. at 11.25 twist barrels but the M24 weapon system with .300 win. mag. barrels is 1-10 twist.
A ton of variables and options that could be used.