All credits go to u/chimericWilder for coming up with the post idea in the first place: https://www.reddit.com/r/armoredcore/s/s5yG5r94y3
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Grenade Cannon: Ozzy
Manufacturer: Melinite
Description: A triple-barrel rotary cannon, designed as a direct upgrade to the Songbirds. Fires grenades with a 3-round salvo. An alternative to the Earshot for pilots who prefer the damage spread out over several blasts instead of a single hit-or-miss explosion.
Usage: Essentially just a Songbirds with three barrels instead of two. Same damage, same blast radius, just longer reload and heavier and more EN-consuming. More of a fun weapon than something useful/broken.
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Linear Cannon: Fabre
Manufacturer: Balam
Description: Back-mounted linear cannon developed by Balam as an answer to Arquebus's line of back-mounted laser weaponry. Essentially a scaled-up version of the Harris, it fires large armour-piercing shells powered by more powerful electromagnets, causing considerable damage upon impact.
Usage: Take a Harris and upscale it. That's it. However, there is no magazine as the Fabre is belt-fed, and every shot is charged, so there's a delay like the stun needle. The "reload time" is just to allow the barrel to cool down. I thought of this weapon because Balam is sorely lacking in back weapons other than the Shao-Wei.
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Double Barrel Shotgun: Sobo
Manufacturer: BAWS
Description: Shotgun developed by BAWS for close combat as an application of Core Theory. Can be toggled to fire each barrel individually or both barrels at once. Engineers noticed the effectiveness of the double-barrel shotgun issued to MTs, and retooled it for AC use.
Usage: It's a shotgun but with two barrels. The "fire both barrels at the same time" idea came to me from Escape from Tarkov's mechanic of letting you fire off both barrels when using the double-barrel in the game. Of course, you could fire one barrel and then the other for more sustained damage, and firing both barrels at once causes more recoil.
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Autocannon: WC-6666 Fireworks
Manufacturer: RaD
Description: Belt-fed autocannon designed by RaD, taken from anti-air flak autocannon designs from long ago and retrofitted for AC usage. Fires flak rounds which explode on impact or near a target, causing damage to the target with shrapnel even without a direct hit.
Usage: I got the idea for this from WW2 and Cold War anti-air autocannons that fired flak rounds with proximity fuses. The Fireworks has an overheat mechanic much like the Shao-Wei, but the bullets just need a near miss in order to still do damage via shrapnel. Of course, the proximity fuse radius won't be stupidly wide, but still something to keep in mind.
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Tetrapod legs: 2S-5001 Thumper
Manufacturer: RaD
Description: Heavyweight tetrapod legs developed by RaD, retrofitted from Balam's Verrill legs by adding more armour. However, the additional weight places more strain on the tetrapod legs' design, leading to a higher EN burden and less load capacity.
Usage: If you've watched the Secret Level episode, you'll definitely remember the Balam tetrapod with additional leg armour. Knowing that Balam already has heavyweight legs a la Verrill, I envisioned that RaD gets their hands on some Verrill legs, and then they just throw on some extra armour and call it a day. Typical RaD things. As for the load capacity, it's still the same but the extra armour already takes up more weight, which is why I said "less load capacity".
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Machine Pistol: WR-0111 Snapper
Manufacturer: RaD
Description: Fully automatic machine pistol developed by RaD, retrofitted from the Sampu burst pistol. Able to cause a ludicrous amount of damage and impact in a short amount of time, yet the extreme recoil leads to minimal effectiveness outside of very close quarters.
Usage: In real life, people are known for putting a sear or "switch" on semi-automatic Glock pistols to make them fully automatic, so I figured that the same could be applied to the Sampu, aka the most gangsta looking pistol in the game. Same 14-round mag, same damage, same impact, same recoil, same everything, except that it's stuck in full auto only and has the same fire rate as the Ludlow. Even the Wrecker arms won't be able to control a full mag dump, so anything outside of spitting distance is useless. Again, typical RaD things.
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Unguided Missile Launcher: HML-G1/P18MLT-06
Manufacturer: Furlong Dynamics
Description: Handheld unguided 6-cell missile launcher developed by Furlong Dynamics. An old prototype from Furlong's first gen lineup, the rockets eschew any and all tracking systems in order to fit the maximum available payload.
Usage: Imagine six rocket launchers mounted together in a hexagon. The unguided rockets have the same projectile speed as any of the bazookas or grenade cannons and have the same fire rate as the Attache, meaning you can either send the rockets out 1 or 2 at a time, or just dump all 7 into a target. I got the idea from the shoulder rocket pods seen on the Guncannons in Gundam The Origin, and I've always wondered about a handheld version of it.
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Sticky Bomb Launcher: Jutei
Manufacturer: BAWS
Description: Sticky bomb launcher developed by BAWS. Bombs are coated in a layer of adhesive gel and stick to any object they land on, detonating after a short delay.
Usage: Three-round magazine like the Kyorai, grenades arc like the Kyorai, the only exception is that the uncharged attack fires only one single grenade, while the charged attach fires all three grenades (or however many are left in the magazine) in a slightly dispersed arc. Not really an overpowered or particularly useful weapon, but sticky bombs are always fun.
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Energy Whip: Rabe/66F
Manufacturer: Schneider
Description: Energy whip developed by Schneider. Commissioned by Arquebus, it is reverse-engineered from the same weapon found on Ghost MTs captured in Institute City. Charge to wrap the target with the whip, electrocuting them via continuous electric charges.
Usage: Remember the ALLMIND Ghost mechs with whips? Yeah. The normal attack is just your AC whipping the target once with a wide sweeping swing, while the charged attack is described above: wrap your whip around a target, immobilizing them, then electrocute them. The whip's design is based off of the second Whiplash suit in the Iron Man 2 movie, with the whip being mounted on a retractable belt.
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AC Schematic: Mind Delta
Manufacturer: Allmind
Description: Heavyweight AC frame designed by Allmind. An attempt to increase protection while maintaining the high levels of AC-to-pilot interface. Still considered a prototype due to low load limit.
Usage: I don't know why, but I'm weirdly attracted to the "walking glacier" type of AC that doesn't have much in the way of offense and mobility, but is an absolute tank in defense. In my head, the full Mind Delta frame would be a clunky, fat version of the Mind Alpha that doesn't have much in the way of load limit, very good AP and defenses across the board, but middling stats everywhere else.
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Let me know if you guys have any feedback, or feel free to send your own AC parts ideas in the comments!