Couldn’t deku dodge nagants bullets with his finger smashes?
In my opinion? Probably not.
If you were to ignore the VS side of things... we don't have much to go off for the muzzle velocity of Nagant's bullets, but it's very clear that Horikoshi has written them in such a way that they aren't normal bullets.
Being able to cross hundreds of kilometers worth of distance in mere
seconds is not something that I think just falls into the supersonic range of velocity. Even people on SpaceBattles are saying that a bullet going that fast would have to be moving at hundreds if not thousands of times the speed of sound to make contextual sense.
You could ignore that, and just say that's an outlier compared to everything else, but...
Nagant's bullet was able to reach Deku almost instantly, from three kilometers away from his original position, even before he was able to react properly with Danger Sense.
That is an object crossing thousands of meters worth of distance in what is effectively a fraction of a second.
And even if you assume that this action took place in what should be a pretty fair, if not slightly negligible assumption of Deku's reaction speed ~120 milliseconds, it still clocks in at around 25,000 m/s or over
Mach 70.
Even being conservative and going, "Well, maybe Deku was just caught off guard and didn't get the time to react," effectively disregarding the implications of the story... at best you'd still have to admit that the bullets crossed that distance in under a second, due to the nature of Deku's incredibly impressive reactions, because there's no reason to assume that someone who can
air kick a ******* bullet has reaction time slower than a normal human's.
But again, let's be conservative and say that it took place in a single second...
That still means the bullet is traveling at over ~3,000 m/s or a little bit over
Mach 9.
Deku's finger smashes, if we adhere to the laws of momentum and kinetic energy conservation, should have more then enough force in them to accelerate Deku's body faster than those speeds, but at the same time, adhering to the same laws would mean that Nagant's bullets are tens of thousands of times faster than sound.
Going off of how his finger smashes actually function in the series...
There's not much blowback or recoil at all.
Considering how much momentum he is flicking that finger with, to part the water like that, he should go flying through the U.S.J's roof... but he doesn't. Probably because in Horikoshi's mind, Deku's finger smashes don't transfer that much momentum to Deku's body or because he's never considered such an event occurring.
Realistically, in the context of Horikoshi's 'vision' of how much his finger flicks propel him, he shouldn't be capable of moving more than subsonically if we're generous and assuming he's flying aerodynamically with Float and Black Whip to aid him.