TexasCHLforum.com

Gents,
Gonna pick up a spikes 9mm lower here shortly. Curious as to if any one has built an AR9. What uppers fit the spikes . I know some can be finicky even though they all are supposed to be mil spec.

I’m not certain that there is a mil-spec for 9mm lowers. I do have a "multi-caliber" Spikes Gen II Billet lower, and it will fit all of my uppers, including 2 from BCM, and 4 others I built myself—including 2 that use the matching Spikes Gen II Billet upper.

So my guess is that pretty much any 9mm upper will fit your 9mm Spikes lower. I think the bigger issues are probably things like proper selection of the BCG and other moving parts.