John - thanks very much for this. This is helpful. I stuck my barrel dimensions in with your table. Seems like my Firing Pins CTC is pretty close to your measurements, but my barrel width compared to your Width at BF is off more. I have some pretty wide barrels. I can't see having a BF width that was considerably less than the barrel width, so I'm guessing around my serial number they had a bit wider frame. If the original frames for my barrels had a BF width closer to your dims, then there would have been a step that would not be desirable.
I still don't think this fully confirms if I could fit these barrels to a 16ga frame or not, but I'm leaning toward not being able to. The wider barrel width might suggest they could fit a wider 16 ga frame, but I would expect the firing pin CTC to be wider for a 16ga.
JE Flues 20ga, #182974 , 2.013 , 0.968 , 1.382 , 1.370, 1909
JE Flues 20ga, #208430 , 2.070 , 1.041 , 1.373 , 1.315, 1911
JN Flues 20ga, #249772 , 2.165 , 1.090 , N/A , N/A , 1914
JE Flues 20ga, #280972 , 1.903 , 0.977 , 1.366 , 1.332, 1917
JE Flues 12ga, #182124 , 2.355 , 1.121 , 1.498 , 1.438
JE Flues 12ga, #202500 , 2.410 , 1.187 , 1.496 , 1.422
JE Flues 12ga, #358962 , 2.333 , 1.155 , 1.448 , 1.425
JE Flues 12ga, #377767 , 2.315 , 1.121 , 1.453 , 1.411
Looks like I'm probably sticking to finding a suitable 20ga frame and not looking for 16 ga frames.
Thanks,
Jason
