I'm sorry that some are frustrated that there aren't interesting links, but not everything is on the Internet.
The media loves to tell the story of the unarmed fighters ready to (presumably give their lives to) ram the jetliners. But they actually have two weapons at their disposal.
TP rounds are real bullets
And peacetime fighters carry them for a variety of reasons. However, they will only fire if the gun is mechanically and electrically enabled, which cannot be done from the cockpit - if the ground crew did not enable these, the cannon would be inoperative.
A bullet is just a chunk of metal moving fast.
A combat fighter-jet round is not a bullet; it’s a shell like an artillery shell, effectivly a flying hand grenade. It includes a device called a fuze which makes it detonate on impact or flight time. The bursting charge fires, blowing shrapnel in every direction. Just what you need to down enemy fighters. These jets didn’t have any of those.
The Vulcan cannon recovers its empty shell casings, so even a combat fighter that’s Winchester (out of ammo) has a lot more weight than one with no ammo at all. (again because of the brass) "No ammo" would be undesirable for weight and balance reasons, and why swap ammo in/out every time they go to the range?
They tend to fly with M55A2 "TP" which stands for "Target Practice" rounds, and are mixed with M220 "TP-T" Target Practice/Tracer" rounds. These are cheap, and don’t have the expensive fuze or the bursting charge. But they’re not blanks; they do have a “fast chunk of metal”. In fact, practice rounds used to be painted, so when they swapped paint with a practice target, it could be examined to see who shot the round.
A practice round will do a hell of a lot of damage to a non-jinking, subsonic jetliner. Simply putting a random hole in an airplane is unlikely to crash it; that’s why shells are needed in dogfights. But with a jetliner, the fighter pilot would have time to line up accurately on an engine and hit it with enough practice rounds to wreck it. (At small risk to shrapnel damaging the fighter, but the pilot can eject).
Or could focus on a control surface, such as the elevator or its operating mechanism.
Ramming is not necessarily a suicide gesture. It was often used as a tactic in war by pilots who had no intention of dying.
It is a matter of ramming sturdy points on your aircraft, such as weapons hardpoints, landing gear mounts, or even drop tanks; against fragile points on the other airplane. Pilots are well familiar with these points.
Such an attack could be made at great advantage against a jetliner, which has no rearward visibilty at all, and could not anticipate the attack.
Also from comments (Thanks WPNSGuy):
Actually, the jets are generally loaded with 20mm TP. Weight and balance. Non explosive, non incendiary. But will fire just like combat level rounds. In the F-16, 510 rounds. The issue is the gun is hard wired safe by an external device, that can only be removed from outside the cockpit. In the interest of time on that morning, I do not know whether the ground crew removed that on those 2 jets.
[The safety lock] is not complex, but the access port is right next to the intake. If engine is already running, not a place to be taking things apart. And generally, it is safety wired.
And again, thanks WPNSGuy:
I can't find any specific published references for jets being loaded with 20mm TP all the time. But, in 16 years as an armament spec, 9 years specifically on F-16, I can't think of a single instance when a jet was loaded with non-fireable dummy rounds. Yes, fiberglass duplicates, weighted appropriately, exist. But we didn't fly around with them. And unless the jet was on a specific training mission for the gun, it was mechanically safed with the 'holdback tool', and/or electrically safed by having the actual power cable disconnected.
And thanks too to Charles Bretana, who says:
I have 2000+ hours in a USAF fighter (The F-4C/D/E Phantom). The F-4E had an internal gun. And we ALWAYS carried 640 rounds of 20mm TP ammunition. The only thing necessary to arm the gun was to pull a safety pin out of the gun assembly before we took off. Why you ask? Why not load "Dummy" rounds instead? Why? Then for any training mission the arming crew would have to offload the dummy ammo to load the TP. And Logistics would have to manufacture these dummy rounds and distribute them, Why do that when we already had thousands and thousands of TP rounds available?