This is a great gun,but I think you are useing wishfull thinking about group sizes. Generally the regular Bushmaster (or colt or RR ect) with the chrome lined barrels and 5.56 chambers will shoot 1 1/2 to 2 " @100 yards with good ammo. Regular nato grade 5.56 55gr fmj (Q3131 or m193) will be 2"+ @ 100.
This is providing you have a great solid rest,good optics,and a skilled hand on the trigger. New guns will usually have fairly rough triggers that take some time or work to smooth out.
I like to think I am a decent shot on a good day , my bushmasters with aftermarket RR national match trigger , a good set of sandbags,and a great high power Leupold scope will shoot XM193 type 55gr into 1 1/2-1 3/4" at 100yd and winchester 45gr VARMINT (great stuff) into about 1 or 1 1/4" ten shot groups @100yd. Some of the black hills stuff will about match the win 45gr. Logic tells you the groups should be twice the size at twice the distance but when shooting this doesn't always hold true.
Now almost anyone can throw a couple lucky shots into a nice 3 or five shot group,but when you try to shoot 3 or 4 ten shot groups in a row things always grow bigger.
Let us know how you make out. Get a really good scope and solid mount,consider an aftermarket trigger (I like the RRNM as a fairly cheep,easy to install,nothing to adjust great trigger) and head to the range!
Chrome lined is the way to go for any general purpose rifle but its not for show that most ar's built for competition or varmint use have something other than chrome lined.