I’ve had an RS HK416A5 castle nut knocking about in my parts box for a couple of years and I didn’t expect to be writing a blog about it. But since featuring the A5, people have been asking, “Why the extended castle nut?”

It’s also a feature of the HK416A7, so it’s clearly not going away:

Initially I didn’t know the answer to this, so it was time to do some research. As luck would have it, because the castle nut (or receiver extension nut) is one of the enhanced features of the A5 over the 416D, the answer was on H&K’s website.

It’s a question of strength:

Receiver extension to stabilise and accomplish drop test criteria under real conditions

As usual with genuine, made in Germany H&K parts, the quality of the item in all its glossy black glory is sublime.

Read more in the Do You Like Koch? series:

Review: Heckler & Koch Quick Release Captive Rear BUIS

Review: Heckler & Koch HK416 Slimline Stock

Review: Heckler & Koch Assault Grip

Catch up on more HK416A5 news, here:

Fifty Shades of RAL8000 Part 1

Fifty Shades of RAL8000 Part 2

Fifty Shades of RAL8000 Part 3

Fifty Shades of RAL8000 Part 4

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