Anyone running a BCM upper on a Spikes lower? Do they fit well together?

Should be fine. BCM doesn’t mill their own upper and lowers. They just assemble them. Believe it or not sometime uppers and lowers from same brand have shitty fit.

Both of my rifles have complete BCM uppers mated to stripped Spikes lowers filled with yummy parts. First one was put together 2-3 years ago and the second was done about a month ago. Both have excellent fit and finish and function flawlessly. BCM and Spikes forged receivers play very well with each other.

Can’t say about Spike’s, but my BCM SOCOM upper sits on a ‘Poverty Pony’ Anderson lower. I’ve owned the rifle for three or four years now and it shoots very accurately and reliably. There is a tad bit of wiggle in the front pin, but no worse than on an old Colt SP1 rifle I used to own. I have two foam earplugs stuffed into the rear of the lower to tighten it up, but the rifle shoots just fine without them.