I've decided to get on of the AR variants. Three that I was looking at were the DPMS $600, Sig Sauer M400 $900, and Colt M4 $1100. Which one of these would you choose or do you have other suggestions. Also, I was wondering if on the AR variants are the parts are interchangeable or does each company have it's own design. Probably a stupid question but I'm just starting to look at these.