I shot lead in my G36 yesterday. Now, it was only 20 or 30 rounds so it is meaningless as an example but I was at the range to chronograph loads, not shoot mag dumps. Total round count was well north of 100 and the barrel was pristine!
Next time out, I'll shoot a couple of hundred at the action pistol range.
My brief experience with S&S Casting bullets (both 200 LSWC and 230 LRN) has been positive for both the G21SF and the G36. However, I did install a KKM barrel on the G21SF. I didn't need to, I just did it.
Try it and see! Shoot 20 or thirty, disassemble and inspect the gun. Try 50 or 60 more and look again. If you get to multiple hundred and you still don't see fouling, well, you probably don't need a new barrel.
I use the Lewis Lead Remover from Brownell's to look for lead. The flakes show up against the bronze screen - if there are any.
And, BTW, the 200 gr LSWC at 1.250" fed perfectly in my G36.
"No matter how cynical you become, it's never enough to keep up." - Lily Tomlin