Thanks, all. I'll follow up the Bo'ness cemetery lead; and I do have access to all the main civil war soldier websites, such as Fold3, CivilWarData, and also Ancestry - but they don't have everything, and a name like M'Callum could be spelled in a number of different ways by recruiting officers who didn't listen too carefully.
Yes, there was conscription in the American Civil War, certainly after 1863- it was called The Draft, and led to massive and violent riots in cities such as New York. But there are also many records of immigrants, and sailors docked in the east coast ports, being "shanghaied" by unscrupulous people - particularly tavern and lodging house keepers - who would drug their drink, bring in the recruiting officers, and steal the country money which the recruits would have been offered. The next thing the recruit knew was when he woke up in a barracks the next morning.
Bounty money, not country money - spellcheck strikes again...
I checked the four Bo'ness cemeteries on Find a Grave; no M'Callum's or McCallums (except two 21st century ones), but it only records graves with stones, and many of these veterans died in poverty and were buried in unmarkedgraves, I'll see if there's a local history society which can help.