[IYAS9YAS]

Not sure if the reverse is true. The site doesn't mention that.

After looking at more pics, nope, you can't put an AR upper on the Sig MCX lower, no buffer tube accommodation, and the lower appears to have a mount for a swing-away stock where the hole for the buffer tube mount would be on the AR lower.

[Perseverando]

After looking at more pics, nope, you can't put an AR upper on the Sig MCX lower, no buffer tube accommodation, and the lower appears to have a mount for a swing-away stock where the hole for the buffer tube mount would be on the AR lower.

Thanks! Boy, do I feel stupid! Of course you can't use the Sig lower with AR uppers. No buffer tube! I'll put myself in timeout for missing the obvious.