This Reddit thread touches on a persistent point of confusion outside professional aviation circles, but the underlying engineering rationale is well understood among those who work with or study military aircraft systems. The core misconception in the original post is that rotary cannons like the M61 Vulcan (used in the F-15, F-16, F/A-18) or the GAU-22/A (a lighter four-barrel derivative used in the F-35) are heavier and less efficient than a comparable single-barrel autocannon. In practice, the opposite is often true when the full weapon system is considered. Rotary cannons distribute barrel wear and heat across multiple barrels, allowing extremely high rates of fire (typically 4,000-6,000 rounds per minute) without the barrel warping, overheating, or suffering catastrophic failure the way a single barrel would at similar sustained rates. A single-barrel gun capable of matching that rate of fire would need a much heavier, more robust barrel and cooling system, largely negating the weight savings from eliminating the rotary mechanism.
The deeper reason rotary cannons persist, however, is doctrinal rather than purely mechanical. Modern fighter gun engagements are not the WWII or Korean War-style sustained bursts of the past; they are extremely short, high-density bursts fired during a fleeting window of opportunity, often lasting well under a second. A high rate of fire maximizes the number of rounds on target during that brief exposure, which matters enormously given how few gun kills occur in modern combat and how narrow the firing solution typically is against a maneuvering target. Single-barrel autocannons, common on rotary-wing aircraft, ground vehicles, and some naval mounts, are optimized for sustained, lower-rate fire against comparatively static or predictable targets, a very different employment profile than an air-to-air or strafing gun pass at high closure rates.
The F-35's inclusion of a gun at all, and the debate over whether it's even necessary, is itself a live topic in defense and procurement circles, since the aircraft was designed primarily around beyond-visual-range missile engagements enabled by its sensor fusion and stealth profile. The F-35A carries an internal GAU-22/A, while the F-35B and F-35C variants use an external gun pod specifically because internal space and weight margins are tighter on those versions, illustrating that the services still view the gun as worth the weight penalty even in a missile-centric design, largely as a last-ditch self-defense and strafing option when munitions are expended or a merge occurs unexpectedly. This reflects a broader trend in fighter design: despite decades of missile advancement, air arms have been reluctant to fully eliminate the gun, treating it as a low-cost, ammunition-abundant backup system that requires no guidance electronics and cannot be defeated by countermeasures.
For military and former-military pilots, weapons system engineers, and aviation enthusiasts who follow procurement debates, this thread is a useful reminder that airframe design tradeoffs are rarely as simple as "lighter is always better." Weight, reliability, rate of fire, barrel life, and doctrinal employment all factor into a system that has remained remarkably stable since the Vulcan's introduction in the 1950s. While there's no direct operational relevance to civil or business aviation, the discussion reflects the kind of systems-engineering literacy that experienced pilots and technically-minded enthusiasts often bring to online communities, and it's a good example of a seemingly simple design question that actually involves layered tradeoffs across mechanical engineering, tactics, and procurement history.