It is stated that Lady Nagnant's bullets pack a lot more punch than even a Howitzer artillery shell. I will do a short calculation on this.
For reference, a howitzer artillery shell can weigh up to 160 kg, with the shells reach a peak muzzle velocity of 701 m/s. If we plug this all into the kinetic energy equation, we get 39312080 joules of energy. That is equivalent to 9.4 kilograms of TNT. (Small Building level).
Considering the fact that the howitzer gun can only achieve this solely because of it's mass, and that Lady Nagant's bullets are stated to pack way more energy than this by Deku, this would imply that the bullet which hit Deku would have to travel at dozens of times, if not hundreds of times faster then sound. At the very least, I don't see this dipping below Mach 50.
I'll add to this
Her bullets are made out of hair so we can assume they have less total mass and weight than a standard bullet
The most common howitzer that's used globally is the M109A6 "Paladin self propelled, which fires 155 mm rounds, 95 lbs at 821 m/s or 2700 fps
https://www.defence24.com/are-light-howitzers-still-needed-on-the-battlefield
According to this kinetic energy formula calculator here
https://www.calculatorsoup.com/calculators/physics/kinetic.php
The energy output would for 95 lbs at 821 m/s would be 14522643 joules
Now we need to determine the weight of Nagant's hair bullet. We can tell that the round is likely the length of a pen or 6 inches. Also important to note that Nagant's hair is shoulder length or 12 inches
The average human has about 100k strands of hair, also keep in mind that Nagant's hair isn't even shoulder length ei shorter than 12 inches
I split one side of her head hair in 4.5 sections, so she'd have 9 sections of hair total, so we can determine that, if she has 100k hairs the average that to 11k per full section and 6k in the half section
Even if Nagant used an entire full section of hair to form a bullet, 11,000 hairs at 6 inches would weigh roughly 1 ounce
So we can punch in 1 ounce into the Kinetic formula above and we get 32008.472521798 m/s or mach 93
Keep in mind this is on the EXTREME low end as this is only if the bullet matched the kinetic output of a howitzer, however its likely much higher since Midoriya said that Nagant's round hits A LOT harder. This also assumes that Nagant is using massive sections of her hair to make the bullets which is unlikely