HG2022 only gives a damage for pebble rounds with reference to boarders. It doesn't specify what damage it does to anything else (and it gives no damage for normal sand rounds).No where in the rules does it say pebble rounds can only be used vs boarders in fact the fact that we do have a scale says just the opposite it’s just very hard to do enough damage to have any effect.
I don't have access to the earlier MGT2 versions, but MGT1 had different rules.