Found I could only get three 3.5 inch shells in magazine of Nova after shopping for extension on internet decided to make one,went to Lowes for one 1 inch PVC coupling,one 1 by three quarters bushing,one three quarters cap,three quarters pipe you will need 1.5 inch nipple paid aprox $3.00.After you put all this together with cement cut .5 inch off the coupling.Then cut the end off the forearm cap made of hard plastic and grind out flat edge being careful not to damage the threads,slide the coupling over the cap grinding or notching out where barrell meets cap drill and tap with set screws.Can even use stock spring no problem may want to paint flat black before installing on cap.Now 4- 3.5 inch in magazine.