Unarmed F-16 -- why would it happen? As is documented here among other places, a couple of the interceptors sent after United Flight 93 on September 11, 2001 were scrambled so hastily they did not have time for the ordies to visit and arm them up.  While not having live missiles on you at all times makes sense for a variety of reasons -- I can't understand why a fighter jet would be flying around with 0 rounds of ammo for their gun, considering how much of a W&amp;B impact the ammo load has.

Furthermore, even if the media coverage was referring to them being loaded with training slugs instead of live ammunition -- you can seriously damage an airplane with 20mm slugs, as one Grumman test pilot (among other folks, it seems) found out the very hard way!  Sure, you'd have to use quite a bit of cannon fire to do it, but it'd still be preferable to a last-ditch ramming attack, no?

So, how could a fighter be scrambled into a situation where their only option was to ram their target?
If the article is to be believed, the gun was probably loaded with M55A1 training rounds which are a steel core projectile with an aluminum nose-cap used for target practice as opposed to the M56 HEI or M53 API rounds which the jet would go to war with.  If it had either a full or partial magazine of the M55 ammunition, it would have had at least some firepower available to it, but I imagine the crew had resolved themselves to, if necessary to protect Washington DC, ramming their aircraft into the jetliner.  It just goes to show you how utterly unprepared this nation was for that kind of a ruse.  Its obvious that this was an anomalous and desperate measure by two very brave ANG pilots who responded to a call using whatever resources were available to them.

Post 9/11 most fighter bases in the nation keep at least two birds housed in a Hardened Aircraft Shelter (HAS) in a 'hot' condition, that is, fully fueled, 520 rounds, 2 x AIM-9s, 2 x AIM-120s, with the INS aligned and two pilots keeping watch round the clock for the alert.