@inderos:
Just because you can do something doesn't mean you have to do it all the time. Zoro can turn back to normal at anytime, his haki is surely stronger than Usopps, making the bullets fired from Gun-Zoro stronger than any ones Usopp could fire on his own. Also, it would be hilarious… And Starscream is definitely stronger with Megatron in his hand. It would be the same for Zoro and Usopp. Let's not forget Usopp will be the marksman that surpasses Yasopp.
EDIT: Instead, what is Zoro ate the bullet fruit? He could transform back to normal Zoro just before the moment of impact and Oni-Giri anyone thinking he was just a normal bullet.
Even if this were to happen, I'm sure Oda can plausibly make Zoro-Usopp stronger than Zoro or Usopp individually; that's not really my issue here
My issue is essentially forgoing about 700chapters of character development as a sword fighter (learning new skills with the sword like in Alabasta or from Mihawk) to becoming a ranged weapon for another character. Why? It seems like a sort of gimmick (kind of like the reverse in the Enies Lobby arc where Usopp was Zoro's sword…) that would be used just once and not again. (and you seem to have noticed that somewhat by your admission that it would be hilarious...) If that were the case... then Zoro having a DF fruit is kind of contradictory because it's sorta permanent (as far as we know...)
!
The more I think about this scenario, the more absurd I find it..
Gun-Zoro becoming a weapon for Usopp would only serve to make Usopp stronger (by your own admission: it would make the bullets fired by Usopp stronger)
and as for bullet-bill-Zoro getting fired in bullet form and changing back to human form to attack: this is essentially just some method of transporting Zoro to an enemy quickly… it doesn't even serve to add to Zoro's attack. Zoro is definitely fast enough on his own and the added momentum from being shot as a bullet seems nearly negligible.
Basically, in either scenario, the effects on Zoro's strength would be negligible if not negative