Spearman vs Tank Problem - Units by Era

[Ahriman]

But an assault rifle with a modified clip does and it technnicaly counts as a rifle

Nope. That would be the Infantry unit (and an advanced version at that), not the Rifleman unit.
Rifleman unit really represents 19th century infantry. Infantry represents 20th century infantry. And with a handful of exceptions (MP44), no assault rifles until post ww2.