by a lower from bushmaster, rock river arms or any of the other manuf.
then get what ever upper you want.
i have owned 2 of the model 1 sales uppers and they where both sub 1" MOA at 100 yrds.
and i save a bunch of money on them 2
they place that makes the barrels for model 1 sales is called E.R. Shaw.
and they have been in the buiss for i think 30 years.
damn good barrels and as i said before you won't pay an arm and a leg for it.