Roberts bulge tops are sloped at about 30 degrees so definitely penetrable in IRCWCC. The angle makes the balsa a little more difficult to penetrate at range, like sloped armour on a tank.
We had a Roberts in Big Gun once. Its sloped 1/8" armor was effectively impenetrable at range, and it proved effective for several years. Then someone figured out if you get close enough and depress your guns, you can hit perpendicular to the balsa and punch through easily.
