Frame size and barrel regulation

[AmarilloMike]

We all know we can buy a Parker 12 gauge in 1, 2, and 3 frame. I have read here of a 12 gauge that is a 6 frame.

But sticking to 1,2 and 3 frame guns, say all with 30" barrels, how did they regulate them. I have been assured in many online discussions that quality doubles have absolutely straight barrels. So the 3 frame gun is an 1/8th inch wider at the firing pins than a 1 frame gun. So if the bores are absolutely straight then the 3 frame gun's right barrel would point more to the left than the 1 frame right barrel, 3 frame left more to the right than the 1 frame left barrel.

The extra "tow-in" of the 3 frame right barrel is 1/16" every 2-1/2ft (30")

So at 40 yards the 3 frame right barrel should shoot 3" to the left of the 1 frame right barrel (40x3)/2.5)*(1/16)=3". And vice versa for the 3 Frame left barrel.

And then how did Parker regulate that 6 frame 12 bore which has firing pins 5/16" wider than the 1 frame. ((40x3)/2.5)*((5/16)/2)=7.5" So the 6 frame barrels would shoot 7.5" to either side of the 1 frame barrels at 40 yards.