Non-magnifying weapon sights (Baraka, RVN, Combat, Pistol Mini, Kobra and M4-A1 Backup Sights) when attached to a weapon will use the default zeroing distances of the weapon's iron sights, or when it has no sight in the case of the M4-A1 and Longhorn. The only exception appears to be the M4-A1 Carry Handle Sight which functions like normal.
This is especially problematic for weapons like the SG5-K, M4-A1 and CR-75 which have a fixed non-adjustable zeroing distance of 25 Meters when not used with a weapon sight. As a result, attaching any of the stated sights will still not allow them to zero beyond 25 Meters, or whatever the max zeroing distance of the weapon's iron sights may be.
On the opposite side, attaching these sights to a DMR will allow them to zero up to 1100 Meters as this is what the iron sights are capable of zeroing to.