Double Rifle Load Development Shooting Distance

Hi, I've got a new .500 double that I'm having a hard time getting a load that doesn't cross. I've owned other doubles in the past up to .470, and I've never had such a hard time getting a double rifle to uncross. Before I make the assumption its a regulation issue I want to rule out the possibility I'm shooting the gun poorly.

I've been shooting the rifle from sticks at 50 yards, this has been my standard practice for all my doubles. I'm shooting this rifle with open sights.

This...

Double Rifle Load Development Shooting Distance