So after closer inspection, it seems that the B9 is exactly like the B3 except for the main spring being 1/2" shorter. The mainspring and seal in the QF-2 kit should work fine to up the fps a solid 50-100 points as described by B3 users. However, the lack of an anti-bear trap mechanism makes me want to just keep the old spring and not worry about the fps. I'll have to see how low this gun really goes. I haven't heard good things about the velocities it is capable of. A high 300's would definitely warrant a spring change. In any event, I'm almost done with the cleaning (man she's a dirty birdy) and then I'll smooth it up and test it out. If anyone has tested the velocities on a B9 they already have, I would really appreciate some numbers for comparison. Thanks.