Bummer!! Sorry to see that!
Because of their recoil, the stocks on power burners are designed to handle heavy rearward forces, our PCPs don't recoil much and the stocks reflect that with the vertical "pistol grips" being relatively weak.
I like woodworking and have made quite a few slingshots, gun stocks and wood bows. Making a (functional) wood bows gives you a real education in working with the wood grain and I am constantly aware of it - even if it is just a piece of firewood LOL! I am really surprised that a laminated stock would fail like that. As cea1960 said, it looks like the grain orientation was not optimum and should of had the grain direction skewed relative to adjacent laminations like they do in regular plywood.
The stock on a FWB 300 has a reputation for breaking at the grip so the first thing I did when I got one was to reinforce that area with a couple of long screws and some epoxy. If you repair that break you may want to do that as well. Here is how I did it...
To reinforce a (potentially) weak grip I select two #8 or #10 screws of an appropriate length and working from inside the receiver area, using a spade bit, I drill holes deep and large enough to (easily) accept the head of the screw. Next I (carefully to avoid break-out) drill pilot holes a bit longer that the full length of the screw through the area to be reinforced. The pilot hole for the screw should quite large (to minimize stress and leave room for the epoxy) and should just engage the threads - the screws should be quite easy to install. At this point, it is a good idea to protect the working area from epoxy drips with "painters tape". Run the screws in and out to check all is good - they don't need to be cinched up tight, the epoxy takes care of the fit. Clean the screws (with alcohol) to remove any contamination (finger oils and such); then evenly apply the epoxy; insert screws and clean up any excess epoxy. When the epoxy sets around the screw it will make a solid "pin" with intimate contact with the wood. If needed, some epoxy mixed with sawdust (to the consistency of peanut butter) can be used to fill over the screw heads.
Optionally, a wood dowel or a metal rod could be used instead of a screw, I prefer a screw because I feel the threads make for a stronger wood/epoxy/metal interface. Hope this helps someone avoid a broken stock!
Cheers,
Hank