I have a BSF B55 the cylinder of which is pinched at the point the stock mounting lugs are welded to the cylinder.The gun shoots fine with a synthetic parachute seal which is flexible enough to compensate for the pinch at that point in the cylinder, following the contours of the irregularity.I don't know if this is common to all B55's.

I do know the overall build quality of the B55 does not live up to the legend!