You only really 100% need the guns for the drawn out boss fights. The problem is in practice you don't switch to the guns much to use up all of the ammo, and often I find myself using the guns only for the sake of it, just to mix a bit of variety into proceedings, and not because I feel I need to be using them to survive plus it's a bit disheartening to keep coming across ammo box after ammo box and not being able to pick up the loot due to being full up with ammunition already as you haven't been using the guns, so emptying a few random clips here and there helps to avoid that issue. Nor should there be, since this game's unique aspect is the katana + guns experience, and theoretically the developer gets the balance ok IF you were to use up all of the ammo that the game gives you in the chests. There's nowhere near enough ammo to be able to complete the game using guns alone.

Or have some people managed to do a more or less gun-only playthrough?

But as it stands you're more or less forced to use the katana. IMO if the guns were more powerful the game would have more replay value.

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