The P6 Dropstock Black for M4 AEG / 15 Degrees Angle is designed to lower the Stocktube so you can still see through your sight with a full face mask.
This is the biggest problem when you use for example a Dye i4 or i5 mask and want to play with your M4 type airsoft gun. Because the side of the mask hits the stock it is a lot harder to see through a reddot. By mounting this adapter you lower the stock tube which solves this problem.
