My friend, Steve, piloted a F-4 Phantom and he said most versions did not have guns. Those that did had 30mm Vulcan guns. Can't remember what he said, either 1.5 seconds or 3 seconds fire time and you were out of ammo. Of course the F-4 is out of production now. But current jet fighter interceptors may or may not be speced with guns according to purpose.
.TM7
The Vulcan is a 20 mm gatling gun develped for the F-104 and tail gun on the B-58.
During 'Nam when those in combat , with some authority , realized that missiles did not work, real often, and a gun would have meant far more air to air kills, shown by the F-105 which was a long ways away from a dog fighter scoring kills on the very maneuverable Mig 17, this point struck home.
Every U.S. fighter since has had a gun installed and even the Russians did refitting on some of theirs that originally did not have one.
The U.S. still used the 20mm Vulcan but other countries are using guns from the 23mm new , very new Russian guns replacing the older 23 , to the , comparatively , new 27mm Mauser to the old, very 30mm Aden.
When the Brits were still flying the Lightning, oddly the F2 version used by NATO in Europe had permanently installed nose cannons, 30mm Aden , but the improved F6 used by the home defense had had its belly guns removed.
After ' Nam, they were refitted, replacing the fuel tanks that could also be put in that location while over wing fuel tanks, which they had stopped using, were also then refitted.
Hard lesson are sadly the best lessons.