"It's a very powerful weapon but needs a lot of force to be used."
In my opinion, I don't consider using a comma before but because the second clause is dependent.
". Also, it is unclear to me exactly what "needs a lot of force to be used" is supposed to mean, or even what is to be used: force or the weapon.
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In the sentence as you've written it, the comma is not required, but I think I prefer "It's a very powerful weapon, but it needs ...". Also, it is unclear to me exactly what "needs a lot of force to be used" is supposed to mean, or even what is to be used: force or the weapon.
adsfafa comma before but
No, because there no new explicit subject is stated.
adsfafIt's a very powerful weapon but needs a lot of force to be used.
Absent any explanation of what "it" is, I guess you want
It's a very powerful weapon, but it takes a lot of strength to operate it.
Or maybe not.