Re: What do you think?
I read the replies of your fellow countryman and agree that it is a low pressure problem, your brass clearly has not took the shape of your Ackley chambering.
I think that if you have a little extra headspace you did not achieve full(here's a big word) obturation of the chamber, thus leaving the shoulder area unsupported, allowing it to rupture.
Dodgy brass could also be a contributing factor.
I have always been a strong believer in handloading my rounds for fireforming into improved or custom chambers.
With my 6mmPPC bench gun i had no choice(apart from the fact that forming 220 russian is the way to go),but with my 243 Ackley, i loaded new Lapua brass with a medium range charge of fast powder(for that calibre) and a bullet that allowed me to have it pushed hard into the lands and seat nice and tight in the case neck, in this case, a 90grn Scenar.
It may be a little bit of work to go along this route but you can pick the brass you want and the bullet you want and in my opinion this all helps for good fireforming.
"I meant to shoot the pike but the duck got in the way"