In Magna cavalry units can be 2 3 or 4 elements with a 2 element front, so Horse archer in Sk in 2 ranks will always be with a reduced shot. For flexible if in 2 ranks in loose horse archer they can always shoot without reduction.

QRS says for Magna that to shoot at full effect SUGs or TUGs in skirmish shoot at full effect in 2 ranks and 1 rank at reduced effect

Welcome aboard since.
As Nik pointed out they shoot 2 deep at full effect in Magna.

Si