I've got a 12" Centurion rail (on a 16" mid barrel), which actually measures 12.5" from rear to front.
2.25" of rail forward of the front of the GB. If you've a true 13" rail, or even slightly longer, you're still good. From the "Included" bullet-points at the bottom of the product page:
(1) 5/64 X 6" L-Shaped Hex Keys



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