A projectile whether it is a bullet, rocket, or even a rock are effective against lightsabers.
We see several occasions in the lore of objects being thrown at lightsaber-wielding Jedi and Sith, only to promptly be struck aside. By your logic, when Luke cuts apart the metal crates Vader throws at him in Empire Strikes Back, he would be hit by molten metal - that never happens.
Most likely, if any matter survives vaporization upon contact with a lightsaber's extremely high energy yield, it would be harmlessly scattered by that same energy.
The danger a machinegun poses to a lightsaber is the exact same
as that posed by a rapid-fire blaster. There's a reason only Jedi and Sith - people who are trained in the use of the Force - can viably use lightsabers to defend themselves.