Axes were released as an upgraded marker based off the design of the Invert/Empire Mini. Compared to Minis, they use many similar components but almost nothing is directly compatible other than the regulator, air tubes, bolt, firing poppet, foregrip and main circuit board. Performance between them is very similar but Axes have a larger firing assembly which holds more air volume, which allows Axes to operate at lower pressures around 150-psi.
Enhancements from Mini to Axe:
Tank shutoff ASA ("Relay ASA") which is backwards-compatible with Minis
More ergonomic foregrip shape and location
Larger trigger guard area
Trigger microswitch instead of magnetic switch
Tool-free bolt assembly quick release
Firing assembly safety latch prevents the bolt from being removed while pressurized
The following parts are identical between classic Mini and Axe:
Top air transfer tube (although this has been upgraded with the safety latch)
Foregrip housing (although redesigned)
Empire markers use a mixture of both metric and 1/16" AS-standard o-rings. Sometimes you can swap one for the other, but it often requires experimentation because the new o-ring will have a looser or tighter fit compared to the factory o-ring. For instance, the firing poppet's o-ring can be swapped with a 13/90 instead of the factory 2x10-mm ID o-ring.
Classic Minis and Axes use the same Relay ASA regulator. Information on regulator maintenance can be found here: Relay maintenance.