Let's pretend a German machine gun fires so fast it can put 3 bullets in the barrel:
First bullet travels down the barrel because there is tremendous pressure behind it, let's say 40,000psi, but only atmospheric pressure in front of its nose. So the bullet travels forward and gets half way to the muzzle. Pressure behind the bullet is dropping to maybe 30,000psi.
Second bullet starts its journey down the barrel. Remember, First bullet is still in the barrel, with maybe 30,000psi behind it. Second bullet now seals the barrel, with 40,000psi behind it. But it has 30,000psi in front of it! It doesn't travel very fast, does it? By the time the second bullet is half way down the barrel with 30,000psi behind it (and 30,000psi still in front of it because the barrel is sealed and First bullet hasn't cleared the muzzle) and Second bullet virtually stops. Equal pressure on both sides of Second bullet, as long as the first bullet has not cleared the muzzle.
Third bullet now starts its journey with 40,000psi behind it. But Second bullet is virtually stopped. This can't be good.
Of course, inertia will keep all bullets moving. There probably won't be a barrel obstruction. All bullets will clear the muzzle, no doubt. The point is that you cannot have 2 or more bullets in the barrel at the same time without suffering velocity loss. Ballistic acceleration depends upon lots of pressure behind the bullet.