I am building a new AR using a NFA C4 billet lower and NFA side-charging upper. I am trying to find a rail that most closely matches up to the profile of the upper and lower to look as if it was made for that upper/lower combo. I am not as concerned about whether it is a full or partial quad rail or MLOCK but rather how well it transitions from fore-end to upper. With a side-charger I am looking for a clean transition to get quick, unimpeded access to the charging handle. Here is the rail I used on a different brand left side charger build: http://www.cimtactical.com/product/15-slim-quad-rail-sqr/ Just tapping in to the knowledge bank on here to get some recommendations. Thank you all for your time with this. By the way, I haven't decided on a caliber yet. .224 Valkyrie sounds good but I'm not sure about a brand new caliber that might turn out to be a flash in the pan.