A model RockRiver Arms A4 mid-lenght 5.56x45mm, (nothing special) the rear stock has been changed out to carbine length buffer tube with an adjustable shoulder stock. I believe the original buffer was a 5.2 lb and has been changed to a buffer that is 1/2 the length, I am not sure about its weight., should be @3lb so I am told, however I am interested in a JP Rifle, Silent Captured Spring replacement that have different weight options, I wondered what would have been the stock installed weight.