I have been shooting shotguns for 64 years and never seen one I can't shoot right out of the box. Until now.
I just bought a new 12 ga. field pump gun with walnut stock and forearm, and after shooting at 4 targets on station one I noticed that I felt very clumsy or awkward with it.
By station 4 I noticed that every time I pulled the trigger the gun barrel pulled down well below the target. I shot a round of trap and was seriously disappointed.

So I took it home and did some measuring to compare it to my semi-auto and O/U guns.
I have never seen so much negative pitch on a shotgun stock before. So I made a 1/4" shim to increase the toe of the stock moving it out to where it was similar to my trap gun.
It made no difference at all, the gun's barrel still pulled down below the target when fired.

The gun looks just fine, it functions very well jacking the shells and ejecting them 8 feet.
The pistol grip is well shaped pulling the gun into the shoulder as it should.
But I have never experienced a gun that consistently pulls down on every shot?

If someone knows what the problem is I sure would like to know the answer or how to fix.
Mike

The pistol grip

I've been shooting the 870 since the 60's and my experience with pitch is the opposite of yours. The field stock I add a pitch spacer at the heel, trap/tournament stocks are fine as is. Somewhere I read Rollin Oswald comment that the pitch of the 870 is not correct for some....??? You need someone who is experienced in gun fit, can watch you shoot and pick out flaws (if any) in your pump gun ability. IMHO anyone who can shoot the 1100/1187 can shoot the 870 unless they're just not a pump gunner type of person.

If you shoot right handed to hard of trigger pull will cause the gun to go down and to the left and vise versa?

The gun when fired should have the barrel come up, not down.
You probably already know that, so it is not telling you anything.
The recoil pushing back against the shoulder and the barrel
being above the shoulder is what gives the leverage. You
know too, no doubt.

What it all means is the physics of it says what is happening
is impossible, unless the shooter is overriding the tendency of
the gun to move the way it wants to.

It sounds actually like a flinch. Shooters naturally try to
counter the recoil of a gun by trying to push it the opposite
direction the gun is trying to move. That is why is sounds
like the classic flinch.

If you could have someone else load your gun, unseen
by you, it will become apparent if this could be the
problem. When they load no shell and hand it back
you trigger on an empty chamber, if the gun comes
down, you are the culprit doing the movement.
If it doesn't come down, you have a gun that defies
the laws action and reaction and leverage. So it
is going to be quite a mystery.

I am pretty sure you know what you are doing. I just wonder
if we know what the gun is doing. Well, of course we don't know,
why does the barrel go down? It makes no sense whatever.

I wonder what the point of impact looks like compared to the
point of aim. Have you shot it at paper? That may tell you
something, but maybe not.

Maybe it felt strange because ii is new. Maybe also it shoots
way off from point of aim. It is all pretty curious.

Of course this all tells you nothing about why the gun just
felt strange to you.

Maybe you should have some others try shooting it, maybe
a lightning bolt will come down and someone will see the
light.

You will figure it out pretty soon and then tell us about it.