I am just commenting on what I saw. He did have the rifle looked at by a competent gunsmith to no avail . He has since sold the rifle. I did not shoot it so I cannot say if it would double for me or not.

You are correct in that they may have an inertia type cocking for the second shot (so if your first does not fire - you are screwed - maybe 450 can say if his is this way), - and it did double quite often. We surmised that the recoil caused the rifle to move back, releasing the tension on the trigger, the second bbl would cock and then the bounce back forward would cause his wandering finger to hit the trigger again.

In any event he sold it and went back to a Steyr bolt rifle and he is happy. I liked the rifle otherwise - if it had double trigegrs I would buy one.

As an aside, I too had issues with my new Merkel 470 doubling and people told me it was operator error. Especialy when I had 2 good smiths look it over. One smith even changed the front trigger to an articulating type to give my big finger more room. He was depressed when it still doubled when I pulled the rear trigger first - it doubled that way all the time! The second smith, when I sent it back (he builds doubles btw)tore it all down and with a magnifier found a very tiny burr on a sear or whatever (I cant recall) It took some effort to work it out due to the hardness of the metal and voila, no more so-called operator induced doubling . Made me feel better too that I was not a klutz.

Point is, I guess you just never know with a rifle. Expensive, cheap or anywhere in between.