How does object rotation work?

Question

Game development

Hello everyone
I'm trying to solve the problem of raising a new weapon by a player (I import 3d objects from a blender). According to the idea, after raising the barrel of the weapon, it should look towards the mouse along the y axis (3d game with the camera setting on top, like in alien shooter). But in the end, the weapon looks a little in the wrong direction. Please tell me how to fix it. The code in which the player picks up a weapon:

Vector3 mPosition = Camera.main.ScreenToWorldPoint(Input.mousePosition);
        Vector3 pos = mPosition = player.transform.position;

        Quaternion initialRot = weapon.transform.localRotation;
        GameObject prevW = player.GetComponentInChildren<ShootingScript>().gameObject;
        GameObject w = Instantiate(weapon, prevW.transform.position, Quaternion.identity * Quaternion.Euler(0, pos.y, 0));
        w.transform.SetParent(prevW.gameObject.transform.parent);
        w.name = "Gun";
        w.AddComponent<ShootingScript>();
        Destroy(w.GetComponent<WeaponRotationScript>());
        Destroy(prevW);

here

Quaternion initialRot = weapon.transform.localRotation;

the weapon object (this is a processed prefab), zero coordinates in position, rotation, unit scale
ps
and in general, I try to do this:

GameObject w = Instantiate(weapon, prevW.transform.position, Quaternion.identity * Quaternion.Euler(0, 0, 0));

but in the end the Gun object is rotated along one of the axes. Tell me, who knows why it might be so
pss
I just tried to apply the Quaternion.Euler(0, 0, 0) function, without multiplication, the situation does not change, the rotation vector is still different from zero