Chuck --
In response to your posting directly above ....
Now, this problem is getting really interesting -- if confusing.
I undertook your suggested evaluation by coating the barrels lightly and uniformly with oil and viewed the sides of the barrels against a uniformly lit, blue, sky. There was no obvious "swoop" at the muzzle for either barrel. But, the barrel profiles could obscure a slight swoop.
Pursuing the point, yet, further, I undertook the following evaluation. I have extended Briley chokes for the gun. The extension for the chokes is about 0.5 inches in length. And, indeed, the extension of both choke are round by caliper measurment, as would be expected based on the machining techqiue used to fabricate them. They show no measurable difference in diameter over their extension length, i.e., they have no taper, +/- 0.0005. The seperation between the chokes at the muzzle appears to have been designed to have an approximately 1 mm gap. Placing a 0.0385 shim between the choke extensions, it just snugly fills the gap between the choke extensions. This shimmed choke configuation can then be viewed "side on" against a back light, to determine whether there is a taper in the shimmed gap - between the shim and the extensions -- from the muzzle to the end of the choke extensions. The answer to this experimental question is that I can discern no taper in the gap. Based on using the calipers as a gauge, I think I should be able to discern a taper in the gap in this shimmed configuartion, if it were 0.001 to 0.002 inches or larger. If a lack of parallelism between the chokes, alone, was the cause of the 16 inch divergence, I should see an increase in the gap of 0.0055 inches from the muzzle to the end of the choke extensions. ( 16/ 1440 X 0.5 inches) Obviously, I did not find this result.
I then undertook an additional evaluation. re: barrel to barrel alignment. Placing the barrels on a cushioned support, I position the barrels, such that I could view the apex of a tall tree against the sky at approcximately 40 yards distance. The barrels were positioned and then fixed, such that the apex was centered in the view through the upper barrel, using the concentricity of the muzzle and chamber image rings for eye alignment. Without touching / moving the barrels, I viewed the apex through the lower barrel along the axis of this barrel in the same manner. The apex was centered in the view. The two images were not discernably different. This result indicates that the bores of the barrels are approximately parallel.
Interesting ..... if confusing. So .... now where ....
Thus, far I have not considered gun dynamics (recoil) before the shot charge leaves the muzzle. There is an intrinsic difference in torque and barrel movement generated by the upper and lower barrels in association with recoil. This would provide for a difference in upward barrel rotation from upper to lower barrel -- the upper barrel producing a larger rotation. But, I have always considered this effect to be very small. On the other hand, I have never tried to qualtitatively estimate this difference. If this effect is a contributing cause, the differential in the upward movement of the muzzle during recoil would have to be a none trivial fraction of an inch. On the face of it -- this seems to be a very large differential movement in the short barrel transit time for the shot charge. I could attempt to make such a calculation, but, it would take some time to characterize the parameters needed for even rough estimates. Do you know whether anyone has made and published such estimates? I suppose I could use some 3 inch - 1 1/4 oz loads in point of impact pattern testing. This would magnify the differential in the point of impact between the barrels -- if recoil is a significantly contributing parameter.
So, to date .... Not much progress in providing for an understanding of the origins of the problem. Interesting, though.
Best Regards,
Don
