The setup on a AR weapon means that with a Magpul PTS CTR or MOE stock it is comfortable but some may need a cheek riser on it to allow the comfortable cheek weld while using optics.
The problem arises when you go to cycle the charging handle. As the AR has quite a bit of backward extending travel, a stock not set to the fully rear position while it has a cheek rest attached may block the reward travel of the handle.
This cheek rest mounts on to a Magpul PTS CTR or MOE stock to be used just like a cheek rest of high optics but when you go to cycle your bolt now the bolt can keep traveling and push the cheek rest back because this one is spring loaded to slide. Now you have a cheak rest that doe not block the cycling of the charging handle.
While obviously not so important on an AEG, on GBBRs which have full travel like the real steel this riser could prove just as useful on a GBBR as it can be on the real steel.
Polymer cheek riser and metal mounting bracket and screws for use with Magpul PTS CTR or MOE stock (stock not included).