Mega armour is a suit of massively thick and heavy armour plates over a powered exo-skeleton. Though slow mega armour has the advantages of giving a 2+ armour save and includes a shoota and power claw. However, an Ork in mega armour always moves as if in difficult terrain (but there's no extra penalty if actually moving through difficult terrain). If you have a unit that includes several Orks with mega armour just make one roll to see how far the Orks get. Mega armour weaponry may not be changed for other types (because it's built in) but the shoota may be upgraded to a kombi-shoota and/or a kustom shoota. An Ork in mega armour may not use the following abilities, equipment or weapons: jump packs, bikes, infiltration, bioniks, frag or krak stikkbombz or tankbusta bombz.
Errata — Chapter Approved: Codex: Orks Q&A (v1.0): In a unit with a mix of mega-armoured and unarmoured models (eg a Warboss without mega armour leading a retinue that includes mega-armoured Nobz), each model moves at its own speed — this is simply the normal rule for units where some models are in difficult terrain and others aren't, since mega armour makes its wearer move as though through difficult terrain. There's no need to slow the whole unit down to match its slowest member, nor to give the unarmoured models the mega-armoured ones' penalty.