Against ARM 20 you're looking at 2 damage unboosted and 6 to 8 damage boosted. Against ARM 18 you'll get 6 damage a round unboosted and 12 to 14 damage boosted.
A hunter gets you 3 damage unboosted vs ARM 20 and 6 or 7 boosted. Against ARM 18 you get 4 damage unboosted and 7 to 8 damage boosted.
That's not really diminishing returns. Sure it's more focus intensive. But a pair of minutemen can utterly waste some heavies in a single turn.