Look at the direction of the grain in the pictures posted by M.D. No wonder it needs a reinforcing rod. The stock blank was cut fairly square with the grain direction. There was a similar problem with the TC Patriot pistols. A rifle stock cut that way would be likely to break as well. Check the grain direction on most rifles. Most likely it is at a slight angle to both the length axis and width axis of the stock so that some continuous layers of grain runs the length from the lock area through the wrist to the butt stock. If a rifle or shotgun was cut square like that, it would likely break at the wrist. I have cut several stocks from planks, but always made sure the stock was not square to the grain direction.
A pistol stock would be far stronger if the layers of grain were more vertical to the length axis of the stock.
The grain direction is something rarely discussed in gun stock making. I came across a discussion about this concerning the TC Patriots on another forum about a decade ago. It had something to do with the factory using straight cut planks instead of quarter sawn planks (or vice versa). A certain percentage of the stock blanks had the grip cut square to the grain as in the photo posted by M.D. I also came across a similar discussion concerning why so many late 19th century Belgian muzzle loading shotguns were cracked or broken at the wrist.