I NEED SOME ADVISE. BASICALLY, I'M IN THE MARKET FOR A PRE-BAN AR-15 CARBINE, BUT KNOW LITTLE ABOUT THEM, AND NEED HELP ON WHICH TO BUY. I FOUND A WEB SITE, WWW.PREBANARMS.COM, WHICH HAS A PRETY GOOD SELECTION OF AR-15's. THE A2's GO FOR AROUND $3,000, WHILE THE SP-1's GO FOR ALMOST A GRAND LESS. BOTH ARE DESCRIBED AS "UNBLOCKED". WHAT EXACTLY IS THE DIFFERENCE BETWEEN AN A2 AND AN SP-1? IS IT WORTH $1,000 BUCKS? ALSO, IS "UNBLOCKED REFEREING TO F/A CAPABILITY? NOW, IF YOU GUYS AREN'T SICK OF ANSWERING QUESTIONS FOR ME YET, I GOT ONE MORE FOR YA. IS THERE ANY DIFFERENCE BETWEEN AN R6001, AND A R6520? I THINK THOSE NUMBERS ARE RIGHT. I APPRECIATE ANY INFO YOU MIGHT HAVE FOR ME.