The Aero Precision XL Upper Receiver is forged from 7075-T6 aluminum and features an enlarged ejection port, the XL Upper Receiver is your best choice when you're planning a build to send big rounds down range. FeaturesPort door opening enlarged to enhance larger diameter brass ejection (.450 Bushmaster, .458 SOCOM, .50 Beowulf, etc.)Features Laser Marked "XL" designation on the upper receiver tangCompatible with AR15 Lower receiversCompatible with AR15 Port Doors and Forward Assists.250 takedown pin holesLaser engraved t-markingsIncludes port door and forward assist