By the way, you don't need a fancy dial bore gizmo to measure a 410.
A serviceable Chinese telescoping gauge set is about $20.
Mind you, this is not LS Starrett quality but it works just fine for this. Mine says $19.95 and I'm pretty sure it came outta Harbor Freight.
The trick is to measure the bore just behind the choke (or just before the recess if it's screw choked), torque it down and let it slide back down and out the breech (only necessary if there's choke, obviously). All the .410's I've measured (and admittedly it's only a handful) allowed the gauge to slide all the way back down and out the breech.
When evaluating the actual constriction of a screw choked barrel, don't forget the 'jug' effect of the necessary choke inlet. They all have this to some extent, and it's not a trivial factor. Thus, we find interchangeable chokes systems that shoot tighter than marked. A .020 12ga for instance after the jug effect is considered becomes a .025 or tighter. It's not just the modern wadding that makes new guns shoot tighter than the old timers.
