Strike Industries drop-in replacement buttstock for AR15/M16 carbine uses mil-spec sized receiver extension tubes. The Viper CQB stock integrates features sought by military professionals and firearms enthusiasts around the world.
The ViperCQB stock is very much like the MOD1 but it is even more compact and features open access for AEG users.  In addition, the stock features QD cups to allow for use with QD slings.

Strike Industries Viper CQB Buttstock for M4 AEG / GBB
Polymer Construction
With QD Sling Swivel mount on Both
Suitable For M4/ AR Style AEG/ GBB Rifle (Mil-Spec Buffer Tubes)