Geissele HK MR 5.56 Rifle Trigger Assembly 4.75 lbs Black. After installing the MR 556 trigger you must cock the hammer in order to engage the safety unlike the HK trigger system where you can place the selector on Safe with the hammer forward. Specifications for Geissele Hk Mr 5.56 Trigger: Manufacturer: Geissele Color: Black Trigger Shape: Curved Trigger Pull Weight: 4.25 - 4.75 lb Trigger Pin Size: Standard Mil-Spec Pin Hand: Ambidextrous Gun Make: Heckler & Koch Gun Model: Heckler & Koch MR556 Gun Type: Rifle Condition: New Caliber: 5.56x45mm NATO Country of Origin: USA Package Contents: Geissele HK MR 5.56 Rifle Trigger What firearm models does the manufacturer say Geissele Hk Mr 5.56 Trigger is compatible with? The manufacturer says that Geissele Hk Mr 5.56 Trigger is compatible with Heckler & Koch MR556Geissele 05-220: HK MR 5.56 Rifle Trigger Assembly 4.75 lbs Black