Both of these should work pretty well on AKA 870s.

Infil density: 10% is good enough ( Please note that since the walls aren't thick in the first place, 10% basically equals 100%....you will see )

DON'T use brims. Brims can cause failure to eject, just use skirt or nothing.

870_shell_5.stl:

• Much more durable.
• More printing time
• Decent muzzle velocity
• I suggest using this one if you are not in a rush.

870_shell_4.stl:

• Less durable
• Less printing time (probably only decreases by about 15 mikes / shell)
• I suggest using this one if you need a huge amount of ammo in a short time (e.g. wargame tmr)