I am gathering parts to build another upper and possibly a whole new rifle.

I bought a Bravo Company upper receiver on the EE.  I test fitted it on both of my Rock River Lowers, It fits pretty loose.

You guys that have bought bravo uppers and bravo lowers, how is the fit?

Also, you guys that have either bought Rainier Arms upper/lower combos or a RA upper and lower separately how is the fit?

ETA: How about a Spikes upper and lower fit (forged, not billet)?  Mega upper on mega lower (non-billet)?

Fit will depend totally on the particlar upper & lower combo that you happen to have. The fit of one shooter's combo will most likely not be the same as yours. You will only know for certain once you have a particular set in your paws to compare.

Tolerances vary. My Bravo upper won't even fit into my new RRA lower that I have. I have a Bravo lower coming in..