The attachment point for optics on the SPAR-16, SPAR-16 GL, and SPAR-16S are misaligned with the rail on the weapon. The optic sits too high on the rail, with the attachment points clipping into the rail rather than locking to it.
Also, optics on the SPAR-16 are mounted very far forward. This might come down to preference, but I find it a little odd.