I'm in agreement that using the one gage horizontally you will have flex, how can you not. Even by holding onto that ring, how do you know you are holding it the same the farther you go.
Oct 2008, I started making a horizontal one, but had it supported on both ends, one end was fixed and the other end went into a fit on the top and lower rod. Indicator traveled on a square tube. Took free end off , inserted barrels, then put free end on and zeroed indicator on that side. Lifted needle and moved to far end and made sure it was zeroed there. You would have to zero it on the ball bearing at both ends.
Sounded great, except I forgot about the choke section.
Now have another idea, same principle, except on the lower rod drill for a 3/8" ball bearing to be inserted on both ends of the rod the barrel rests on. This would take car of any choke and any unforeseen areas in chamber. The barrels would only be resting on two points. The muzzle end would be on the fixed end and far enough away from any choke area. The breech end could be made so that if using barrels between 26-32" that it would read all.
On the free end the square could be filed so that it slipped in easily, and on the rod end it could be a .002 larger and both could be secured with a wing nut. On the fixed end a set screw would do.
Using 3/8" rod with a 3/8" ball bearing, would give you about 9/16", good for 10, 12 or 16 ga. Any thing smaller and you might have center flex, maybe.