With modern automatic weapons, if their targets obligingly massed in a single spot, sure. Bolt-action rifles, less so; Civil-War-era muzzle loaders, still less.

So, do you know offhand exactly how many soldiers were killed by other soldiers in all those conflicts? Do you know how fast and effective those weapons were? Do you know what the distribution and skew of killings per soldier are and how that changes from conflict to conflict? You do not know any of those factors, all of which together determine whether the Marshall estimate is plausible.

'Marshall made everything up' is a good argument. 'Look, there's lots of dead soldiers!' is a terrible argument which is pure rhetoric.

Note that, to make up for an increase from 25% to 55% of rifle-firers, as is claimed from WWII to Vietnam, artillery would have to become twice as deadly just to remain on an even footing; this seems to me unlikely, even though there certainly were technical advances.

Ceteris is never paribus. You're just digging yourself in deeper. Those conflicts were completely different - WWII and Vietnam, seriously? You can't think of any reasons artillery might have different results in them?