The standard load is 78gr IMR4198 and a 275gr bullet. My understanding is that it was an aftermarket custom action. If you look closely at the barrel, 2 copper colored burnish Mark's can be seen about 2 inches and 3.5 inches from the breech. This is where the 2 bullets were at ignition. They always rupture just behind the projectile. Most do not split the action to the point it fractures in half but this is the first custom action I've seen double loaded.
We have discussed destructive testing in the past but it cost money. If anyone wants to do some destructive testing, I am willing to supply the single use strain gage which will cost me $40 each plus a $25 cable that will likely be destroyed too. My pressure test equipment will record chamber pressure via a PC. The expensive part is the barrel and action. As I stated before, I have traced to 118kpsi without a failure.
Black powder and substitutes require less than 1000 psi to maintain semi explosive burn after ignited. Some require no pressure at all. This is why the bullet must be fully seated against the powder or damage could occur due to expanding gasses slamming into a bullet some distance down the barrel.
Smokeless powder is not the same, in that it must have several thousand psi to maintain semi explosive ignition. The initial pressure is supplied by the primer but with the bullet a distance off the powder, no pressure is captured. I actually can tune my SML load by seating the bullet off the powder which lowers chamber pressure. I use a lock collar on the ramrod and shoot groups at increments of .025" off the powder. Once I get to .100" off the powder, I increase my charge 1 gr and start over so as to keep my fill percentage high above 95%.
A barrel obstruction can cause blow up but with the smokeless powder used in this blow up, a bullet on the powder would have to be there too. IMR4198 or any smokeless powder in its burn rate range or slower will not detonate with the bullet very far off the powder. Powders in this range require a constant 5000 psi to continue to burn at a semi explosive rate. This cannot be obtained without the bullet at least close to the powder. There would have to be a second bullet in the barrel as an obstruction. Think about metallic cartridges. Most are not 100% case filled. I like at least a 95% case fill but some are down around 90. Just the other day, a friend had a hang fire in his 450 bushmaster Ruger American. I ran the load in Quick Load and found it was only an 81% case fill. When he aimed down, the powder was .200" or more off the primer. We seated the bullets .100" deeper and used an over powder wad for positive ignition.
Barrel damage can occur even in cartridge guns by using a slow powder with a very light for caliber bullet. What happens is the charge goes off and the light bullet outruns the pressure wave. Then the bullet begins to slow down only to have the pressure wave catch up. One fellow devised a load that would blow the last 3 inches of muzzle off his rifle.
In SML's, a different phenomena can happen. A ml has much less bullet to bore friction than a rifle because the bullet does not get engraved by the rifling as it exits the barrel. This allows the bullet to outrun the pressure wave. I have seen secondary pressure spikes near the end of the barrel that are equal to or slightly higher than the primary chamber pressure. Long ml barrels with large doses of slow burning powder are where I have recorded this. Such as 40cal charges in excess of 110gr of RL16 with a 275gr or 300gr bullet. It shoots tiny groups but the pressure trace is scary.
I hate that it happened to the fellow but as I understand, he has not given up.
This is why I don't build guns for others. Occasionally I will sell one of mine and when I do, I stress over and over and over to use a marked ramrod. Typically, I make a collapsible ramrod specifically for that particular gun. It is such that when inserted unloaded, the entire ramrod goes into the barrel with only the handle sticking out. The ramrod is also marked so the mark is visible at the muzzle when the gun is loaded.