I was just reading through this thread to post this exact suggestion.
The bore of a 12 gauge is .729" (not backbored), and the choke is basically based upon how many points of constriction at the muzzle. A US dime is .705" in diameter, so if the dime will drop through the barrel, it's not a "full choke" barrel. Since your concern was damage from buckshot through a full choke, drop the dime and if it passes, move on to patterning.
As was stated previously, try different loads, with/without buffer, and even the low recoil LEO loads, you'll find each gives a different result. Try different size buckshot too, you may find a smaller size does better at your 30 yard distance. It's not as expensive as testing slugs can get, but if you know you want a certain size pattern at a certain distance, then find the load that gives you that.