I have an older shield so I tried a little experiment. I put the performance center upper on my old lower and it still dose the same thing. on either lower I can pull the slide back by hand slowly, and toward the end of the slide you can feel a gritty resistance like metal scraping metal. My old shield feels smooth all the way no resistance. So I put the performance center barrel in my old shield. and the old barrel in the performance center upper. and the old barrel has no gritty resistance in the new upper. but the performance center barrel in the old upper has the gritty resistance feel like metal scraping metal no matter which upper i put it in. So thats making me belive it is the barrel. EDIT- Also my new performance center barrel has a rough and scratched surface were your arrow is pointing. and my old barrel with 2000+ rounds through it has no marks or burrs at all on that part.