First of all, let's address the issue of the manufacturer:
The firearms manufacturer can tell a LOT from the physical evidence, so don't try second-guessing whether or not they'll be able to tell what happened.
Combine the physical evidence with the known history of the gun, which I'm assuming you gave them the details of, and they'll be able to piece together a reasonable idea of what happened and why.
The "why" may not be a single-point-cause-of-failure, though. However, it's a given that, properly used, no firearm should suffer a catastrophic failure like the one you've indicated. So there is likely one overriding cause.
I'm pretty sure they've got people working for them that have a pretty good understanding of Fracture Mechanics and that those people will be tasked with examining the firearm accordingly.
The ammo manufacturer, however, does not have the hard product evidence that the firearms manufacturer does. It's physically eliminated when you pull the trigger and the round goes off. However, they MAY be able to tell something from the cartridge remains. Less likely, though, than the firearms manufacturer will for a firearm.
BUT knowing the details of the history of the weapon, including the types of ammunition it's used, the chokes installed, and the amounts of ammunition cycled, along with the presumptive care the firearm received betweeen shootings, the ammo manufacturer can make an educated assumption about their ammunition. And knowing the lot number for the ammunition being used at the time is a great help as well. I rather imagine that they track any issues with their ammunition by lot number, and as such a record of firearms related incidents may indeed clue them in to a problem.
BOTTOM LINE:
You should definately contact both companies (as you've already started doing).
The absolute WORSE thing that can happen right now is that both the firearms manufacturer and ammunition manufacturer can tell you that there is nothing they can do for you. That's your worst fear at this point and if this is what happens...well, you're out one nice gun but still happy to be able to count to ten using both hands and both eyes.
HOPEFULLY what will happen is that one (or both) companies make at least a good-faith gesture in your favor, the extent of which will make you happy.
You don't know what will happen until you pursue the companies with your problem. For my own experience, I've had two problems over the last 3-plus decades of shooting, both involved ammunition. One was Winchester .22 WMR, where one cartridge had a creased case and the other was Federal .22 WMR, which had a fantastically noticably hot round that split the case. In both instances, I wrote the ammunition company with the details of my problems. And, in both instances, I got a free box of ammunition from them in response, with a letter apologizing for any inconvenience.
If that barrel is similar to a 391 barrel, I bet everything has to be 100% right for them to stay together; the 391 barrel is pretty thin compared to other shotgun barrels.
