The 7.5 cm automatic rifle (75 mm, 3 inches) was produced in various incarnations through the history of projectile cannons. The Nazis developed a semi-automatic 75 that was mounted in heavy fighters and interceptors to more effectively shoot down Allied bombers during WWII. The concept remained active, but in a reduced prominence, relegated to gun based anti-aircraft systems. Many of the rapid fire heavy guns were made, and many were quite good. But they were maligned, seen as poor cousins to the more technologically advanced and often dramatically more expensive missile based systems. Expensive things like missiles fell out of favor during the Second Dark age, and guns became the go to for weapons. When the first mechs entered production the autocannon, the mainstay of tanks for centuries, was not favored. The guns were large, and the recoil and vibration of the weapon could sometimes cause stress fatigue in the machines mounting them.
A second generation mech, the Sentinel was a rather mediocre machine with average speed and performance and it teethed no new technology. Rather it was a cost cutting machine, introduced to replace several older machines in the arsenal of the Great Lakes Republic while employing extensive cost cutting measures since it was not intended to remain in active service for more than a decade at the most. The Sentinel toyed with hard point mounting systems but didn't make the technological leap of the 3rd generation of mecha. It did, however, mount a fully automatic 75mm gun in place of an arm mount. The Sperry-Colt performed flawlessly, producing a hailstorm of 80-90 rounds per minute and was extraordinarily robust and reliable. The Sentinel proved itself to be a top notch fighting machine and it proved that in the modern era of lasers, particle cannons and missiles, the gun still had a place on the battlefield.
The Autocannon and the 3rd Gen Mech
The Sperry-Colt company took the reliable 75 mm autocannon back to the drawing board as the the Federation 3rd gen mech program (Iron Soldier) was going into effect. The loader and recoil systems were made self contained, hard points for electronics were built into the gun as well as an integral laser sighting system. After 14 months of work, the end result was the Sperry-Colt Whirlwind. The Whirlwind is just the sort of weapon that mech pilots wanted, reliable and fast shooting, but accurate at short to intermediate range. Unlike other guns being designed with ammo hoppers or internal bins for ammo, the Whirlwind has a magazine like a standard assault rifle. Handling the Whirlwind in a mech is functionally identical to field stripping a rifle bare handed. The magazine for the gun holds 30 rounds, and is moderately armored to keep if from being ruined by an errant round or shrapnel hitting the casing. At 700 pounds, the magazine requires either a hoist, crane, or mech to be picked up an moved.
The standard bearer of the Atlantic Federation is the Michigan designed and built Wolverine main battlemech. It's standard weapon is the Whirlwind autocannon system, and the two machines were literally made for each other. The Wolverine is fast and heavily armored, and the men and women who pilot the brutal mech are the sort ready and willing to charge into close quarters battle (less than 1 miles in battlemech terms) and let fly with their big gun. While most mecha can withstand a single hit from a round as relatively small as a 75mm, they are usually hammered with three to five rounds in a close grouped burst. The Whirlwind is also a mutli-role gun and can fire special munitions, such as red tipped explosive armor penetrators, yellow tipped high explosive, blue tipped depleted uranium, or white tipped tungsten penetrators. There are also green tipped proximity rounds for dealing with low flying aircraft and helocraft.
The Cosmic Era AC/5 versus the Battletech AC/5
One of my biggest complaints about Battletech was that the designers knew apparently goose egg about actual military hardware, especially when it came to how heavy most of it was. The Battletech AC/5 weighs as much as the entire gun turret on a modern Destroyer in the Navy, but has a range that is more suited to WWI. I have traditionally compared the AC/5 to the 75mm to 90mm range of guns, and the rounds for these guns are certainly heavy, but closer to 20 to 30 pounds each, rather than the 100 pounds dictated by Battletech. The Whirlwind has a maximum range of near 5 kilometers, it has a moderate to short barrel and emphasises rate of fire over long range accuracy. It's effective range is around 2 kilometers.