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HECKLER & KOCH RECEIVER EXTENSIONS

The receiver extension on top is an HK416 commercial spec tube and the one on the bottom is an MR556 mil-spec. As you can see here, the commercial spec extension is longer than the mil-spec extension.

HK 416 A3 Commercial Receiver Extension VS HK MR556 Mil Spec Receiver Extension

HK A3 Commercial Buffer Tube


Length - 199.07mm or 7.834”


Diameter - 29.68 or 1.167”


Top to bottom measurement - 37.20 or 1.466


Weight - 144.5 grams or 5.10 oz




HK Mil Spec Buffer Tube


Length - 183.12 or 7.209”


Diameter - 29.07mm or 1.145”


Top to bottom measurement - 36.91 or 1.453


Weight - 104.1 grams or 3.67 oz


The receiver extension on top is an HK416 A3 commercial, the bottom one is a mil-spec MR556.

HK 416 A3 Commercial Receiver Extension VS HK416 A5 Commercial Receiver Extension

HK A3 Commercial tube


Length - 199.07mm or 7.834”


Width - 29.68 or 1.167”


Top to bottom measurement - 37.20 or 1.466


Weight - 144.5 grams or 5.10 oz




HK416 A5 commercial tube


Length - 199.07mm or 7.834”


Width - 29.68 or 1.167”


Top to bottom measurement - 37.20 or 1.466


Weight - 143 grams or 5.5 oz


The receiver extension on top is an HK416 A3 commercial, the bottom is an HK416 A5 commercial tube.

A5 vs A3

Each of these receiver extensions have a raised area on the bottom that acts as a stop and prevents the castle nut from turning counterclockwise once it reaches a certain point, indicated here by the red arrows. 

This is important because the newer extended style castle nuts are much larger and without adequate room to back the castle nut off, the buffer retainer assembly cannot be installed. As a result the extended style castle nut cannot be used with the A3 style commercial receiver extension.

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