I did have this at one time, was long ago though...

I think the easiest thing to do would be look at the apron hooks on the playfield. If you can either adjust them so the playfield sits a touch lower, or just put a shim under them and you should be good. Shouldn't take much.

Easier than messing with the canon assembly, that thing pretty much is what it is if I remember right.

I had this problem when I first put my laser cannons in. I turns out the playfield was not full set into it's locked position. My playfield was sitting about 3/8" too high and therefore the domes were touching the glass. Once I set the playfield into it's proper position, problem solved.

QSS

I've got the "Domed" after market cannon covers on my Sttng and have never encountered this problem before. The only rational way the glass could be touching is that the playfield is not seated properly!!