In my last post about the Bear River Sportsman 900, I fixed a intake leak, reduced headspace, and ended up with a rifle that shot on par with a stock Daisy Powerline 880. Kind of disappointing, since I really like the handling characteristics and accuracy of the Sportsman 900. I’m not one to give up, so the story continues here.
I tore down the Sportsman 900 once more, determined to squeeze out the power potential that I know is there. As I mentioned previously, the mods I had done opened up some valve volume. To take up some volume, I took a 3/8″ length of some 3/16″ round stock and placed it inside the spring. This gives enough room for the valve to open and reduces volume allowing pressure to build quicker:
I reassembled and got no more velocity than before. There’s foolery at play and I’m going to get to the heart of it. As mentioned in my previous post, I hadn’t done anything with the two dimples in the pump head. This time, the pump head was removed from the piston, chucked in a power drill, and ground down against a belt sander. Once the dimples were gone, I polished the face with a piece of fine sandpaper against some glass.
This further reduces headspace, but not enough to account for the lack of power. I then turned my attention to the rubber bumper between the piston and pump head. It is a very soft material, and I suspect it is giving me grief. In its place, I inserted a nut that was ground down just enough to provide tension against the pump head pin:
I went ahead and replaced all o rings before I put it together once more. Ten pumps were put in and a 7.4 grain wadcutter loaded. This time, I got what I was after!
I gained nearly 85 fps and 2 FPE over my previous mods using the same pellet. The shot to shot velocity is very consistent. I haven’t tested heavier pellets yet, but I suspect they’ll produce even more energy than these light ones do. We are now looking at reliable squirrel and rabbit hunting energy.
There are many other things I’d like to do with these rifles. I just might have to pick up another refurb or two!