I always figured it was because of the rail... Who knows. Because I would use an M4A1 variant or MP5 in a CQB. :P

I think there is two criteria, but I'm not a R/S fanatic...

1. The slide needs serrations near the front.
2. The company needs a new marketing 'niche'.

I'm sure the rail has nothing to do with it, because the R/S Wilson Combat CQB Elite comes in two flavours - w/rail and w/o.

Its just Wilson's name for it, as a reliable backup in CQB
Nothing really past that