If you have a Lead Sled that will help you solve any pulling problems when patterning. JW nailed it. Start with the light loads at close range. Because your pattern is so tight it will give you a very good idea of where to start with your new sight. Make adjustments to your sight at close range and then move back.

As to the 835 that another poster mentioned shooting to one side....I had similar issues with a Mossberg and heard of others having problems. Possibly the fact that they are cheaper made. Being over-boared really helps them pattern extremely well but they are erratic.