Trying to put together a AR. I bought the DPMS lower with m4 stock from command surplus.com today. When I was looking for uppers today I noticed the lower I bought say 16 inch. Does this mean that I can only put a 16 inch barreled upper on it? found a nice 20 inch flat top and wanted to do that so I'm not stuck with the carry handle that does not come off. Bye the way that's the only other upper I found for a good price. Also how is the lower handled for transfer. Set it up for ffl transfer but do I have to register the whole rifle when it's finished. Thanks