I'm having a hard time imagining how a Mossberg 500 could FTF due to anything short of the shell being loaded backwards or something else very out of the ordinary. I'm thinkin' you could pack the action of a Mossberg with axle grease and still pump rounds through the thing. Surely the Mil-Spec tests put that Mossy into more torture than a simple 21 degrees.
I don't know a thing about Glocks. But when any gun FTE's, I'm lookin' at the ejector(s) for crud/grease/bug poop etc.
Sgt Lumpy - n0eq