I understand all of this, and that the finger slipping onto the rear trigger will cause a doubling, however, this is my problem:

When I pull the rear trigger of my Merkel .470 model 140, 9 times out of 10 the right bbl will fire. BTW -- I certainly dont use a life round in the right chamber, but a primed case only. But, the beast has doubled on me more then once. It did not do so when I hunted with the rifle, but it does when I practice over shooting sticks. A very good and experienced (in doubles) gunsmith cannot figure this out, he says all looks fine mechanically in the rifle.

When my first shot is with the right bbl - all is fine, and the left bbl will not double.

Any suggestions would be of a great help.

btw -- this is agreat sight!