Here are the measurements from the Oscar Quam insert. Thanks to Henry H for the reference drawing.
Measurements:
A=4 inches
B = 2.5 inches
C = 1 inch
D =1.8 inches
E = 2.250 inches
F = the insert did not have the original reed but a 2.3 inch reed works and just reaches an old pencil mark on the insert
G = about 0.740 inches
H = unknown but apprears to have been squared off at tip
I = tapered to 1/4 inch near tip.
J = 0.358 inch average
K = 0.730 inch
L = 19/64 straight reamed
M = 0.140 average
P = 1/8 inch per foot
Hope this helps if you wish to recreate one. The overall length of the barrel is 3.5 inches. See Ben's tutorial (under duck call tutorials) on how to make the barrel.
Fred