With polygonal barrels - consensus is "don't do it". At least a couple manufactures that sell poly barrels suggest "bad things" may result. (copper after lead).
The whole "shoot FMJ to clean out the lead" thing is a myth.
What it really does is lay a layer of copper *over* the previous layer of lead fouling.
Try cleaning a firearm where someone has shoot lead bullets, followed by FMJ. You'll wind up pulling alternating layers of lead fouling (black patches) and copper fouling (green patches) out of the bore. For best results you'll need to keep swapping between lead remover and copper remover as well.
+1Been doing that for 25 years. Never had a problem. Just blows it right out.
Been doing that for 25 years. Never had a problem. Just blows it right out.