The Spare parts list, Table 3 (gas system), shows the Break Action Magazine damper and codes it as Part # 299W. Original .308 barrel. Clean with "Wipe Out". Oil the bolt and rotating head. I've completely dis-assembled the gas system after the issue began to clean the two pieces of the Piston Extension assembly.
Use 168 grain cartridges, Barnes and Hornady A-Max. 165 grain Remington Sirocco. Federal 180 grain Trophy bonded.
Test fired 15 cartridges last week and the bolt does not remain open after the last cartridge is fired so existing issue not resolved.
I purchased the Break Action Magazine damper from the Canadian source for Benelli firearms.
Haven't a picture at the moment but shown in part breakdowns on USA web sites.