looking for the right mag extension

[Irons Man]

I have an HK M1S90 I want to find a mag extension that wil most closely fit or match the length of my 18 inch barrel

Do I need a +2 extebsion or +4?

My gun does not have the plug in it

I assume that the +2 wil bring me to 7+1 mag capacity

Is this rght?

WHere can I get one, I know they are now expensive but I will pay anyhow

[ChrisP]

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