My experience with NIDs shows that the early guns, like the Flues before them, can be a little off in barrel length. I've got one that Walter has vetted as being in the records as 30-inch and it is actually 30 3/16 inch. That is the most off I've found. Also, it has been my experience that the NIDs from 1926 to 1935 that I've run a bore mic in have been overbored while those from 1936 on have been true to gauge and dead on the even barrel lengths.