* WARNING * NOT YET FINISHED
Make sure to comment your ideas in the comment section!

Rotary Autocannon:
20mm to 40mm round size
Variety of ammunition types - will list all i can think of at the bottom of this section
Propellant length customization (for more powder behind the round)
Rate of Fire from 600-4000rpm (Adjustable for all round sizes)

Custom Ammo Belts - select what rounds you want on your ammo belt

For custom ammo belts - say you want a gun that can fire many types of ammo without having to switch them up in the designer. say you want to fire Armor Piercing Shrapnel , High Explosive Frag Incediary and Semi Armor Piercing High Explosive, you could use a fully customizable ammo belt to do that. Example:

Belt 1: APS-HEFI-SAPHE-HEFI-APS
Belt 2: HEFI-HEFI-SAPHE-APS-APS

Get what i'm saying?

Ammo Types (Some of these i made up to be more effective against certain targets) will have numbers at the end of them for which round size they can be used on. for example, HEFI 20mm means HEFI but it can only be used on 20mm. Round types will be abbreviated, so Armor Piercing would be AP, High Explosive would be HE and so on. any round with Tracers will have -T at the end.

Armor Piercing - ALL Sizes
Armor Piercing, Composite, Ballistic Capped - ALL Sizes
Armor Piercing Composite Rigid - 30mm, 37mm, 40mm
Armor Piercing Shrapnel - 37mm, 40mm
Armor Piercing Discarding Sabot - 40mm
Flak Shell - 37mm, 40mm
High Explosive - ALL Sizes
High Explosive Fragmentation Incendiary - ALL Sizes
High Explosive Anti-Tank - 40mm
Shotshell - ALL Sizes

Flak Shells explode after a certain distance. This can be set in the designer.
Shotshell ammunition is a shell casing, then a tube with propellant filled with shot that airbursts at whatever distance is set in the designer.
Armor Piercing Discarding Sabot is basically shooting a smaller, higher velocity round out of a bigger gun :)

Cannon: 37mm-183mm
Cannons are to be used on vehicles like tanks, large armored cars and some aircraft. Shell sizes, propellant length and round type can be adjusted in the Designer. You can have up to 8 round types and you have to fire or interrupt your reload to load a different shell type. Reload times are fully adjustable from 0.1s to 40s (historical accuracy or balls to the walls op, am i right?) and all guns feature adjustable recoil. (realistic or op man)

Cannon Sizes: 37mm, 40mm, 45mm, 47mm, 50mm, 57mm, 75mm, 76mm, 77mm, 85mm, 88mm, 90mm, 94mm, 95mm, 100mm, 102mm, 105mm, 107mm, 114mm, 120mm, 122mm, 125mm, 128mm, 130mm, 150mm, 152mm, 155mm, 183mm

Cannon Shell Types: In-Game, the shell types will be abbreviated to shorten the menu size. Use the Action Groups to figure out what ammo you're using.

Anti-Concrete - 150mm, 152mm
Armor-Piercing: ALL Sizes
Armor-Piercing-Composite: ALL Sizes
Armor-Piercing-Composite-Ballistic-Cap: ALL Sizes
Armor-Piercing-Composite Rigid - ALL Sizes
Armor-Piercing-Discarding-Sabot - 76mm, 90mm
Armor-Piercing-Fin-Stabilized-Discarding-Sabot - 90mm, 105mm, 120mm, 152mm
Armor-Piercing-High-Explosive-Ballistic-Cap: ALL Sizes except 183mm
High-Explosive-Frag - ALL Sizes
High-Explosive-Squash-Head - 90mm, 105mm, 120mm, 183mm
High-Explosive-Anti-Tank - 50mm, 75mm, 88mm, 90mm, 105mm, 122mm
High-Explosive-Anti-Tank-Fin-Stabilized aka CHEAT-FS - 90mm, 105mm, 122mm, 152mm.
Flak Shell - 75mm, 76mm, 85mm, 88mm, 90mm, 105mm, 120mm, 128mm

Autocannon: An automatic cannon that doesn't rotate and accepts larger caliber rounds.

Round sizes: 20mm-100mm
Accepts all rounds that Cannons do, except HEAT and HEATFS. Also includes:

High-Explosive-Fragmentation-Incendiary
Semi-Armor-Piercing-High-Explosive

ALL SHELL TYPES FOR EVERY CANNON: AP, APC, APCBC, APCBC-HE, APCR, APDS, APFSDS, HEF, HEFI, HEAT, HEATFS, HESH, FLAK, SHOT, SAPHE.

MACHINE GUNS: Rapid-fire small caliber weapons designed for infantry support and aircraft armament.

MG Round Sizes 5.56mm - 14.5mm
5.56mm,7.62x39mm,7.62x51mm,12.7mm,14.5mm

MG Ammo Types:
AP - Standard AP ammunition
Ball - just.. Bullets.
API-T - AP round with incendiaries and a Tracer
AP-T - AP round with a Tracer

NOTE: Non-Tracer and Non-Incendiary rounds shouldn't start planes on fire unless it hits an engine or something like that