John;
With the gauge marking showing the bore to be .410" & you measured .411" with the gun having been kicking around for over 100 years I'd say that's mighty close. Sounds like you did good job measuring there at least. It seems many German built guns had fairly deep rifling & given this one was probably built for lead bullets the .426" would be total rifling depth of .015" (both sides) which sounds about right.
I like that folding tang peep sight on it.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra