Looking thru Magpul they apparently have 2 sizes available...I have no idea what to order, pls help, here's their note:

*NOTE: There are two basic sizes of M4 Carbine receiver extensions (buffer tubes): Mil-Spec and Commercial-Spec. If you have a Commercial-Spec extension on your rifle, make sure to order the Commercial-Spec CTR.

Specifications based on standard-length (flat-back) M4 Carbine receiver extension and may vary depending on tube manufacturer.

Friction lock performance may vary depending on the host receiver extension's dimensional tolerances.

You want the Milspec buttstock for your 15-22. Milspec receiver extensions are a tad smaller in diameter than the Commercial ones are.

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SIC SEMPER TYRANNIS