best slug ammo for 20 GA m2 with rifled barrel

[Swilliam400]

Recently purchased a Benelli m2 slug gun chambered in 20 GA it came with the fully rifled barrel I am trying to find a slug that it likes without spending a bunch of money it shoots the Hornady 250 gr mono flex good at 50 but only can get about a 4 inch group at 100 

 

hopefully someone can shed some light for me thanks 

[remarkable]

I have worked on several m2 20 Gauge.  They perform very well with Remington Accutip 3" or 2 3/4".