A friend was helping me unload the truck upon returning from a shoot, and my shotgun slid out of it's case onto the toe of the butt, sending two eight inch long splits up the buttstock almost parallel to the bottom line of the stock. He was sick. I said not to worry...no pieces missing and clean cracks.
I clamped the gun in rubber pads in my machinist's vise, spread the crack as much as I dared, and hydraulicked Elmer's carpenter's glue down the cracks as far as it would go. I clamped the stock closed, wiped off the glue that squirted out of the cracks with warm water on a cloth, and let it cure. Finally, I scraped off the overflow, light sanding, stain to match and some tung oil to re-finish. The cracks are invisible. A little bruise on the horn butt plate on this old double is all that remains of a catastrophe.
Elmer's Carpenter's Glue.