Blackhawk's Buttstock Mag Pouch is compatible with all standard fixed buttstocks and carries most rifle magazines. It attaches to buttstock by adjustable straps and has an elastic froward slip protection strap on back. The lid is adjustable for 20 or 30 round mags, and it is made of 1000 denier abrasion-resistant nylon and webbing. This model has a black finish and is compatible with your M4.
Specifications:

Type	Buttstock Magazine Pouch
Color	Black
Material	Nylon
Size	Most Rifle
Model	M4 Collapsible Stock
Finish	Smooth
Model Fit	M4
Mount Type	Buttstock
Belt Size
Shell Count
Caliber or Gauge
Compartments	1