So I'm thinking about building an AR upper, which would be my first time. I ran across this virtual AR builder at Brownells.

http://www.ar15builder.com/

I won't be using that site for my build but thought it was cool. Anyway, so when you guys build a frankenAR, do you worry about compatibility between different parts? Seems like most things are compatible based on my reading. Is this mostly true? I've got a CMT lower and I'm gonna build an upper for hunting. Thinking of going .300 blackout.