THIS would be a quote of note!! FmF pipe fits on bike!!!
It NOT fitting is par for the course.
In your case, it sounds like a placement issue. The pipe can be wrongly placed in the cylinder easily..or it may be tweaked, too (making the fit poor).
Loosen the mounts on the pipe. Maybe take the springs off, too. See if some rotation of the pipe won't improve the stinger fit.
Looking at the joint of the pipe/cylinder, it should be evenly inserted into the cylinder all the way 'round the pipe.
If it IS tweaked, with the pipe off you can 'tweak' it back into place without too much trouble. Insert something long and stout into the pipe (cylinder end), use it as a lever to 'straighten' it back into place.
Even if it IS 18ga. metal.
Try the 'moving it' part before you try the 'bending it' part.