I use a bore gauge...
Shooting steel through IC, LM, and modified chokes, and checking them with a bore gauge, there is virtually no difference in constriction in those chokes since 2007 after shooting thousands of rounds through them (at least through my LM).
Lead or Bismuth will not damage a modern barrel... Both materials are soft and malleable.
Steel and tungsten do have the potential to damage a barrel. The thickness of the wads will generally protect from direct abrasive damage, but as the shot does not compress, it can bulge a barrel... (i.e. trying to fit an object through a hole that is too small)
Large shot sizes (as often used for waterfowl) are more likely to cause the barrel to bulge, as are larger payloads (to my understanding), and obviously damage becomes far more likely with tighter choke constriction. I would think it unlikely that shooting #9's to #6 shot sizes would damage most barrels, but I have heard of such happening with some of the super tight turkey chokes being used...
Often when barrel bulge happens you cannot get the choke out of the gun, and I have heard of the compression damaging threads on chokes....
And also remember, that factory or aftermarket chokes are often not marked correctly... My last two gun purchases both had chokes marked incorrectly, and generally if I order a set of chokes from aftermarket chokes, at least one will be off... When ordering chokes, I make it very clear THAT I HAVE A BORE GAUGE AND WILL BE MEASURING THE CHOKES to make sure they are correct... This honestly does seem to reduce mistakes.
