Swedish Mauser Model 96, in excellent condition, may never have been issued. New to the shooter. Reloads are cast 150 gr Sil GC and 18 gr Alliant 2400 powder, Lapua case, LR primer. Experienced reloader. Fired 3 shots, no indication of problems. Fired 4th shot, and the rifle blew apart. The top of the receiver ring separated and disappeared; the receiver blew backward in the stock and broke open on the right side; the stock came apart at the receiver ring; a big slice of the stock split off, from the receiver ring back to the butt; the trigger group blew out the bottom of the stock. the case head is gone and the case remains in the chamber, with plenty of brass stains around the receiver at the ring. The top and left side of bolt face, and half of the left lug also chipped off.
Once the left-handed shooter returned from the Hand Trauma unit where the doctor removed the end of the right thumb from the quick to the tip, the shooter inspected the target and found four holes. He picked up the pieces except for the top of the receiver ring. The barrel is clear, no bulge or damage. The shooter didn't notice any difference in recoil between any of the four shots, particularly didn't notice excessive noise or recoil on the 4th shot. Later the shooter dropped a double charge into an empty case to determine where the level of powder would be in the case and whether he would likely have noticed that just before inserting a bullet into the case and seating it. Shooter thinks that he would have noticed the high powder level.
The accident appears to be caused by excessive pressure. One of the Lyman manuals (likely Cast Bullets) indicates that 18 gr of 2400 is a maximum charge with a 150 gr cast bullet. Which is more likely - that 18 gr 2400 is an overcharge, that there was a metallurgical problem, or that there was a double charge?