Usopp doesn't have the range to fight Nagant well. He's apparently only got a kilometer or so of range, compared to Nagant have 3 kilometers of range that can all being curving shots that will put him down after one or two hits, and beyond that is hundreds of kilometers (though no curving shots). Nagant scales to above 550 Gigatons (As the bullets put holes in 45% Deku without an issue, ) compared to Usopp who I believe scales to 150.75 Gigatons, an over 3.6x difference which means a single bullet will take off a limb of Usopp.
Nagant has air walk, meaning she can just... go out of Usopp's range, and say "Nuh uh" which means that it becomes impossible for Usopp to actually hit her. She has better mobility with literal flight, and I don't know if Usopp has any feats of beating a HOMING ability through raw skill alone.
While Usopp has more versatility his abilities are... worse then what's needed. Fire Star and Gunpowder Star are brief bursts, Impact dial is only on the area it's stuck, Oil and Sticky star NEED to get near Nagant which won't happen in this gun fight, Pop Green isn't debilitating, Impact Wolf is in a 3 meter radius which can be dodged, etc etc. The only two I can see being useful are Hissatsu, but the only examples of the are at close range, and Flash Dial, due to it being a Flash Bang. However both of them are shown to be at close range.
Nagant has enhanced senses, tracking someone in the storm at night with barely any visuals on him, so smoke and flash won't be able to put her down forever
Nagant only needs one or two shots to kill Usopp same with Usopp, but Nagant will go for the kill immediately and headshot Usopp given that she's the Hero Associations literal Assassin. And with curving bullets, I don't think it will be hard at all.
In the end, Usopp literally cannot hit her if she decides to use air walk and just shoot him from dozens of kilometers away, which is absolutely what she will do because she's not trying to CAPTURE Usopp, she's trying to KILL him,
