The Pentagon’s newest stealth jet, the nearly $400 billion Joint Strike Fighter, won’t be able to fire its gun during operational missions until 2019, three to four years after it becomes operational.

Even though the Joint Strike Fighter, or F-35, is supposed to join frontline U.S. Marine Corps fighter squadrons next year and Air Force units in 2016, the jet’s software does not yet have the ability to shoot its 25mm cannon. But even when the jet will be able to shoot its gun, the F-35 barely carries enough ammunition to make the weapon useful.

The JSF won’t be completely unarmed. It will still carry a pair of Raytheon AIM-120 AMRAAM long-range air-to-air missiles and a pair of bombs. Initially, it will be able to carry 1,000-pound satellite-guided bombs or 500-pound laser-guided weapons. But those weapons are of limited utility, especially during close-in fights.

“There will be no gun until [the Joint Strike Fighter’s Block] 3F [software], there is no software to support it now or for the next four-ish years,” said one Air Force official affiliated with the F-35 program. “Block 3F is slated for release in 2019, but who knows how much that will slip?”

The tri-service F-35 is crucial to the Pentagon’s plans to modernize America’s tactical fighter fleet. The Defense Department hopes to buy 2,443 of the new stealth jets in three versions—one for the Air Force, one for the Navy, and one for the Marines. Versions of the jet will replace everything from the air arm’s A-10 Warthog ground attack plane and Lockheed F-16 multirole fighter, to the Navy’s Boeing F/A-18 Hornet carrier-based fighter, to the Marines’ Boeing AV-8B Harrier II jump-jet. But the F-35 has been plagued with massive delays and cost overruns—mostly due to design defects and software issues. There have also been problems with the jet’s engine. An F-35 was destroyed on takeoff earlier in the year when a design flaw in its Pratt & Whitney F135 engine sparked a fire.

Another Air Force official familiar with the F-35 confirmed that the jet won’t have the software to fire its gun until the Block 3F software is released to frontline squadrons sometime in 2019. Neither Lockheed nor the F-35 Joint Program Office responded to inquiries about the status of the jet’s gun.

Right now, the F-35’s software doesn’t support the use of the aircraft’s GAU-22/A four-barreled rotary cannon. The weapon was developed from the U.S. Marine Corps’ AV-8B Harrier II jump-jet’s GAU-12/U cannon, but it has one fewer barrel and weighs less.

It’s also supposed to be more accurate—when it can be fired, that is. The gun can shoot 3,300 rounds per minute, though the Air Force’s F-35A version can carry just 180 rounds for the gun.

The Navy and Marine Corps versions of the F-35 have differing configurations and rely on an external gun pod. The software won’t be ready for those jets for years, either. And while that gun-pod version for the Navy and Marines carries slightly more ammo, with 220 rounds, some in the military are complaining that it’s not enough. “So, about good for one tactical burst,” the first Air Force official said. “Hope you don’t miss.”

The lack of a cannon is a particular problem, as the F-35 is being counted on to help out infantrymen under fire. (This is known as close air support, or CAS, in military jargon.) The F-35 will lack the ability to mark a target or attack enemy forces in “danger close” situations, said one highly experienced Air Force fighter pilot.

“Lack of forward firing ordnance in a CAS supporting aircraft is a major handicap,” he added. “CAS fights are more fluid than air interdiction, friendlies and targets move... Oftentimes quickly. The ability to mark the target with rockets and attack the same target 10 seconds later is crucial.”

Typically, aircraft will work in pairs where the flight lead will make an initial pass to mark a target with rockets. A second aircraft will then attack with its guns. Incidentally, the F-35 won’t be armed with rockets, either, sources told The Daily Beast.

The reason pilots would choose to use guns over a bomb or a missile is simple. Basically, a pilot might not want to drop a bomb near ground troops in situations where the enemy has gotten in very close to those friendly forces. Even a relatively small 250-pound bomb could kill or injure friendly troops who are within 650 feet of the explosion.

By contrast, a gun will allow a pilot to attack hostile forces that are less than 300 feet from friendly ground forces.

Proponents of the F-35 within the Air Force leadership argue that the jet’s sensors and ability to display information intuitively will allow the stealthy new fighter to do the close air-support mission from high altitudes using satellite-guided weapons. But there are situations where that won’t work.

“GPS-guided munitions with long times of fall are useless when the ground commander doesn’t know exactly where the fire is coming from, or is withdrawing and the enemy is pursuing,” said another Air Force fighter pilot. “GPS munitions are equally useless when dropped from an aircraft when the pilot has near zero ability to track the battle with his own eyes.”

The lack of a gun is not likely to be a major problem for close-in air-to-air dogfights against other jets. Part of the problem is that the F-35—which is less maneuverable than contemporary enemy fighters like the Russian Sukhoi Su-30 Flanker—is not likely to survive such a close-in skirmish. “The jet can’t really turn anyway, so that is a bit of a moot point,” said one Air Force fighter pilot.

“The JSF is so heavy, it won’t accelerate fast enough to get back up to fighting speed,” said another Air Force fighter pilot. “Bottom line is that it will only be a BVR [beyond visual range] airplane.”

That means the F-35 will be almost entirely reliant on long-range air-to-air missiles. It doesn’t carry any short-range, dogfighting missiles like the Raytheon AIM-9X Sidewinder when it’s in a stealthy configuration. One pilot familiar with the F-35 added that “they will not have a large enough air-to-air [missile] load to be on the leading edge” of an air battle in any case.

Another senior Air Force official with stealth fighter experience agreed. “From an air-to-air standpoint, an argument could be made that the F-35A not having a functional gun—or any gun, for that matter—will have little to no impact. Heck, it only has 180 rounds anyway,” he said. “I would be lying if I said there exists any plausible tactical air-to-air scenario where the F-35 will need to employ the gun. Personally, I just don’t see it ever happening and think they should have saved the weight [by getting rid of the gun altogether].”

However, the Air Force official said that very fact the F-35 will not have a functional gun when it becomes operational is symptomatic of a deeply troubled program. “To me, the more disturbing aspect of this delay is that it represents yet another clear indication that the program is in serious trouble,” the official said. F-35 maker “Lockheed Martin is clearly in a situation where they are scrambling to keep their collective noses above the waterline, and they are looking to push non-critical systems to the right in a moment of desperation.”