Just some basics to tighten the discussion:

a.) Suppression is action which causes the enemy not to act through fear of being harmed.
b.) So a sniper rifle can suppress just as well as a machine gun, given a specific context. It is effectiveness versus efficiency.
c.) As an action, "Killing" is what best delivers suppression. Not shooting.
d.) Suppression is not a viable action within itself. You suppress the enemy so as you can do other things, - of which killing him should be top of the list.

All very obvious, but I hope that helps.