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HK416 A1 Receiver Extension VS HK MR556 A1 Mil-Spec Receiver Extension
HK416 A1 HK spec 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 A1 HK spec, the bottom one is a mil-spec MR556.
HK 416 A1 Receiver Extension VS HK416 A5 Receiver Extension
HK416 A1 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 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 A1, the bottom is an HK416 A5.
A5 vs A1
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 A1 style HK spec receiver extension.