Другой способ выстрела: удержание левой кнопки мыши увеличивает энергию выстрела. Чтобы выстрелить нужно отпустить левую кнопку.

from random import randrange as rnd, choice
from tkinter import *
import math

#print (dir(math))

import time
root = Tk()
fr = Frame(root)
root.geometry('800x600')
canv = Canvas(root, bg = 'white')
canv.pack(fill=BOTH,expand=1)

class ball():
    def __init__(self,x=40,y=450):
        self.x = x
        self.y = y
        self.r = 10
        self.color = choice(['blue','green','red','brown'])
        self.id = canv.create_oval(self.x-self.r, self.y-self.r, self.x+self.r, self.y+self.r, fill=self.color)
        self.live = 30
        
    def set_coords(self):
        canv.coords(self.id, self.x-self.r, self.y-self.r, self.x+self.r, self.y+self.r)
        
    def move(self):
        if self.y <= 500:
            self.vy -= 1.2
            self.y -= self.vy
            self.x += self.vx
            self.vx *= 0.99
            self.set_coords()
        else:
            if self.vx**2+self.vy**2 > 10:
                self.vy = -self.vy/2
                self.vx = self.vx/2
                self.y = 499
            if self.live < 0:
                balls.pop(balls.index(self))
                canv.delete(self.id)
            else:
                self.live -= 1
        if self.x > 780:
            self.vx = - self.vx/2
            self.x = 779
    def hittest(self,ob):
        if abs(ob.x - self.x) <= (self.r + ob.r) and abs(ob.y - self.y) <= (self.r + ob.r):
            return True
        else:
            return False
        
class gun():
    def __init__(self):
        self.f2_power = 10
        self.f2_on = 0
        self.an = 1
        self.id = canv.create_line(20,450,50,420,width=7)
        
    def fire2_start(self,event):
        self.f2_on = 1

    def fire2_end(self,event):
        global balls, bullet
        bullet += 1
        new_ball = ball()
        new_ball.r += 5
        self.an = math.atan((event.y-new_ball.y)/(event.x-new_ball.x)) 
        new_ball.vx = self.f2_power*math.cos(self.an)
        new_ball.vy = -self.f2_power*math.sin(self.an)
        balls += [new_ball]
        self.f2_on = 0
        self.f2_power = 10


    def targetting (self,event=0):
        if event:
            self.an = math.atan((event.y-450)/(event.x-20))     
        if self.f2_on:
            canv.itemconfig(self.id,fill = 'orange')
        else:
            canv.itemconfig(self.id,fill = 'black')
        canv.coords(self.id, 20, 450, 20 + max(self.f2_power, 20) * math.cos(self.an), 450 + max(self.f2_power, 20) * math.sin(self.an))
        
    def power_up(self):
        if self.f2_on:
            if self.f2_power < 100:
                self.f2_power += 1
            canv.itemconfig(self.id,fill = 'orange')
        else:
            canv.itemconfig(self.id,fill = 'black')
        

class target():
    def __init__(self):
        self.points = 0
        self.id = canv.create_oval(0,0,0,0)
        self.id_points = canv.create_text(30,30,text = self.points,font = '28')
        self.new_target()
        self.live = 1
        
    def new_target(self):
        x = self.x = rnd(600,780)
        y = self.y = rnd(300,550)
        r = self.r = rnd(2,50)
        color = self.color = 'red'
        canv.coords(self.id, x-r,y-r,x+r,y+r)
        canv.itemconfig(self.id, fill = color)
        
    def hit(self,points = 1):
        canv.coords(self.id,-10,-10,-10,-10)
        self.points += points
        canv.itemconfig(self.id_points, text = self.points)
        
t1 = target()
screen1 = canv.create_text(400,300, text = '',font = '28')
g1 = gun()
bullet = 0
balls = []


def new_game(event=''):
    global gun, t1, screen1, balls, bullet
    t1.new_target()
    bullet = 0
    balls = []
    canv.bind('<Button-1>',g1.fire2_start)
    canv.bind('<ButtonRelease-1>',g1.fire2_end)
    canv.bind('<Motion>',g1.targetting)

    z = 0.03
    t1.live = 1
    while t1.live or balls:
        for b in balls:
            b.move()
            if b.hittest(t1) and t1.live:
                t1.live = 0
                t1.hit()
                canv.bind('<Button-1>','')
                canv.bind('<ButtonRelease-1>','')
                canv.itemconfig(screen1, text = 'Вы уничтожили цель за ' + str(bullet) + ' выстрелов')
        canv.update()
        time.sleep(0.03)
        g1.targetting()
        g1.power_up()
    canv.itemconfig(screen1, text = '')
    canv.delete(gun)
    root.after(750,new_game)
new_game()    

mainloop()