Reamers that I am familiar with are all cylindrical ground on the sides, cutting only on the tapered portion. This includes expansion or adjustable reamers I am familiar with. Did you have to back off the flutes on the side to get the reamer to cut in this manner. It would take extra ordinary force to get a normal reamer to cut on the sides as they are intentionally made not to. This would b quite apt to bulge the barrel in this thin area.

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