G7 vs G1 what's the difference?

[Hatrick]

Foget about that space and measure fom the top of the barrel to the center of the objective and add it to the 1/2 barrel measurement.

 

[Crowe284]

1/2 barrel diameter
+
Gap between barrel and objective
+
1/2 objective diameter

 

[Mike 338]

And what about the space between top of barrel and bottom of objective ?

You add up three separate measurements:
1) Measure Objective Bell and divide by two (center of scope).
2) Measure barrel just under Objective Bell and divide by two (center of barrel).
3) Measure the space between the top of the barrel and the bottom of the Objective Bell. Those two prongs that stick out on the opposite side of your calipers are handy for that as they're meant for "inside" measurements.

Add 'em up.