First thing I would do is get a box of feds and see how well it shoots with them. Some guns simply leave the factory better regulated than others, and the federal ammo is the common denominator with your gun.

Also, chronograph the federal loads. Some guns have tighter chambers, bores, etc... and different velocities are possible with two of the same make, model, etc...

If it shoots well with the federals, then the problem is with your load. If it shoots poorly with the federals, then the problem is either you or the gun.

As this is your first double, it might help to have someone who has shot more of them to run a few through it. Not that you're a bad shooter, but they can be pretty finicky - even in how they are held.

For example one of those thick past recoil pads will move my groups up 1". Also one of my doubles tends to shoot a little low off the bench.

After you and a friend have shot the double, if the groups are still bad (and consistent between shooters), then you might contact Merkel. But I suspect that you'll see the groups improve with fed ammo.

FWIW, the gun should be shot with your forward hand grasping the barrels and that hand only touching the sandbags up front. Elevate the front bags enough that you are sitting up-right with the gun - not hunched over it like you might shoot a scoped bolt gun.

If the feds do shoot well, the work a little more with your laods to get a closer match to the chronod velocity of the feds.

Are you using woodleigh softs in your reloads? The shape an d profile of the bullet can also have an effect on the point of impact.

PS - welcome to the world of double rifle reloading! I assure you that your experience is not unique!