Attacking through a barred forcecage

I've always assumed that the intent of the barred cage type forcecage is to allow trapping a creature while still being able to attack it, but I was just thinking about it and realized there's a problem: the vast majority of attacks will not, in fact, be able to fit through half inch gaps, or will have very poor odds, and given that a half inch gap will stop gases in D&D (gaseous form) there's an argument for it even stopping more diffuse damaging effects, or at least granting cover.

The spell, naturally, is entirely silent on how difficult it is to attack through. So... thoughts?

Personally, I would say that attacking through it with weapons is impossible. The odds of being able to get any weapon, even ammunition from a ranged weapon, through the gaps with any force behind them is infinitesimal. Even the tip on an arrow would be wider than the gap. 

I could see the argument that it is POSSIBLE, though unlikely, so at the very least I would impose a minimum of half cover and disadvantage on any attack through the bars. An AoE effect I would have to evaluate on a case-by-case basis. For example, I would say Ice Storm is completely blocked but Sickening Radiance isn't.

IMO, the gap is too small for any weapon or your Unarmed Strike (I know, Rules Aren't Physics), but technically, if you rule the creature doesn't have Total Cover, you could target it with a ranged spell.