First prove out the new 20mm as a stand alone weapon like the old M-79 40mm grenade launcher. Debug it 110%, then put one "20mm grenadier" per squad, and only then talk about a combined rifle/20mm.
Frankly, I'd love to field test this OICW thing. This is what I'd do.
I'd drag it behind a humvee through sand, gravel, mud and salt water for starters. I'd freeze it and drop it and bake it and bury it.
Then I'd hand it to the Army General in charge of the program and make him fire the 20mm rapid fire after giving him 5 minutes to clean and prepare it under field conditions while tossing artillery simulaters at him for realism. Then I'd get behind a strong wall and watch him fire off the mag of 20mm, if he dared.