When a shotgut bbl bursts "around middle" about 99.98% of the time it is an obstruction of some sort. Most likely around the remaining ragged edges you can see the tell-tale signs of a bulge, which bulged a little too far. Look for a sort of belling next to the burst. A wad could be the obstruction as well as mud, snow etc allowed to find it's way down the bore. The other .02% are mostly mat'l flaws which finally gave way.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra