AR15 STI Lower - Mill Spec?

[riptide]

Perhaps someone here can help me out. Picked up a AR15 STI complete lower the other day for a price too good to pass on.

I've found a 11.5" complete upper that I really like. I'm told that if the STI is millspec dimensions, then I should be good to go.

Where or how do I find this out?

Thanks

 

[hbean]

If by "Mil Spec" you mean will it mate with other AR uppers, than the answer is yes.