Are you looking to see exactly what you have or just trying to see if both bores are the same diameter and have the expected constriction? Measuring gives you a number but a pattern sheet will give you the real picture. There is just no substitute to shooting patterns with the .410.
If I had it to do in large numbers I would use white lithium paste or latex paint to coat a steel plate and either count the pellet strikes on the plate or take photos to blow up on the computer later. Use a circle to find the defined pattern area, 20", 25", 30". I used a cardboard with a circle cut of of it and moved it over the pattern to find the best concentration of pellets. You are looking for the pattern not the poi. 30" might be the norm for most but when patterning the .410 I drop down to 25" or 21-20". 30" might "catch" all the pellets but the effective pattern is much smaller.
After a few dozen patterns you almost can go by first impression just looking at them. Is the pattern even? Is there a lot of clumping or holes in the pattern? Does the point of impact look about right?
After you get that all done then get out your chrono and check you velocity. Speed kills and no more so than when you are talking about pellets. With increased speed you get better penetration. But I found out that you also get blown patterns and a different poi with too much speed. I guess it came down to a point of diminishing returns that after so fast anything above that just give you less results than its worth.
For sporting clays I find that the 1300 plus fps loads are fine but have yet to find a good load to hunt with going that fast in the .410. The patterns all seem a little patchy. Wounded clays are all marked dead.
