When I was about 17 years old I picked up a well used HW35E from a second hand shop. If I remember rightly I swapped my sega megadrive for it After having it for a couple of months I decided it wasn't powerful enough and picked up an OX power spring from my local army surplus store. After much grunt work finally managed to get the bugger screwed back together with said spring and proceeded to test the new beast. A huge kick and a hole through both sides of my dads shed told my it had worked. Christ knows what the power was but I knew nothing in them days. Eventually it did ruin the rifle although I feel it was more user error than spring being pants!!!